In the course of my research for the Flight of the Heron Wikipedia entry I've read various sources about the book and Broster more generally—in literary dictionaries, scholarly books, magazines and various other kinds of publications. Some of these are fairly detailed and pretty interesting, and here are a collection of sources that I hope may be of interest to the fandom!
I've included links to archived copies wherever possible—most of these require an archive.org account in order to read more than a couple of pages. Let me know if you can't access these for any reason—I've got typed-up copies of all the pieces and will happily send any of them to you.
The Horn Book Magazine, volume 5, number 1, February 1929, article by Eleanor Whitney. Short article about Broster, and—most interestingly—a further contribution from Broster herself! This is the only writing of Broster's about her own life and work that I've found, and contains lots of interesting detail both biographical and about her writing process. The ultimate source of this letter is rather mysterious; the Horn Book Magazine claims it was written for them, but a slightly different version of the same letter appears in The Junior Book of Authors in both 1934 and 1951—the latter claiming that it was written for them shortly before her death, which certainly isn't true. Also the Dictionary of Literary Biography (see below) quotes bits from it alongside other quotes not in it, describing the source as an 'undated typescript for a speech' in St Hilda's College archives, and Diana Wallace (also see below) cites something that sounds like this piece as a source for information that isn't in it; which suggests there's yet another, longer and more detailed, version of the same piece in the college archives.
Mrs Beer's House by Patricia Beer, 1968; pp. 115-118, 185-186. This is one of several autobiographies to mention Flight of the Heron as an important part of the young author's reading, but it's by far the most detailed, and it's generally delightful. Beer is the earliest instance I've found of someone definitely recognising the homoeroticism of Ewen/Keith; she also shares our mystification over the short word beginning with S which BPC was going to call Ewen in that one scene, and includes some of her teenage poetry inspired by Loch na h-Iolaire!
Twentieth Century Romance and Gothic Writers, edited by James Vinson and D. L. Kirkpatrick, entry written by E. F. Bleiler, 1982. Not very detailed, but a nice overview, and I agree with their assessment of the ending of The Dark Mile.
Dictionary of Literary Biography, Volume 160: British Children’s Writers, 1914-1960, edited by Donald R. Hettinga and Gary D. Schmit, entry written by Ruth Waterhouse, 1996. This is one of the longest and most thorough sources, covering all Broster's books (except Child Royal, which is inexplicably skipped) and her life in some detail; it's well-researched, using material from St Hilda's College archives, and contains some thoughtful discussion of the books and their themes. Waterhouse says some interesting things about Flight of the Heron, treats Almond, Wild Almond more sympathetically than I'm inclined to—making some decent points, though I can't quite agree—and, somewhat oddly, views both Ships in the Bay! and World Under Snow as entirely comic/parodic works.
The Cambridge Guide to Women's Writing in English, edited by Germaine Greer and Elaine Showalter, entry written by Valerie Purton, 1999. I'm not impressed with this piece, which contains multiple minor factual errors and rather badly misrepresents the themes of Flight of the Heron (yes, gentlemanly values are important, but the one crucial point where Keith despises 'womanishness' in himself it's clearly an incorrect judgement of something good, and I don’t think Ewen frames his gentlemanly values in a particularly misogynistic way. And the characterisation is so not simplistic!).
Contemporary Authors, edited by Scot Peacock, 2003. The writer of this entry gets an impressive amount about the books wrong (did you know that the main characters of The Wounded Name are on opposite sides of the Napoleonic Wars—we're not informed whether Aymar or Laurent is the Bonapartist, sadly—or that Raoul des Sablières is actually an Englishman???) and has relied heavily on the Dictionary of Literary Biography for the rest, but also includes some biographical details that haven't turned up anywhere else, and frustratingly without citing any primary sources.
The Woman's Historical Novel by Diana Wallace, 2005, pp. 7, 29, 33-34. While limited in scope, this is one of the most fascinating sources I know of, and it's therefore particularly frustrating that it doesn't seem to be archived online anywhere. Message me if you'd like a copy of the relevant passages! Wallace discusses the homoeroticism of The Flight of the Heron in the broader context of women's writing about relationships between men and oblique representations of sexuality; she argues that the fantasy element in the book, the prophecy foretelling Ewen and Keith's meetings, gives this forbidden, repressed attraction an opportunity for subtextual expression.
Out of the Attic: Some Neglected Children’s Authors of the Twentieth Century, edited by Pat Pinsent, entry by Chris Clark, 2006. This is the longest and most scholarly discussion of Broster's work I've found, and contains a lot of food for thought. It compares Broster's writing to that of another historical novelist, Cynthia Harnett (I only skimmed the parts about Harnett, not having read any of her books, but I may have to see if I can find them at some point!), and analyses the sentence-level craft of The Flight of the Heron and The Gleam in the North in great detail. Clark articulates a lot of things about Broster's writing style and characterisation that I was vaguely aware of but hadn't laid out for myself this explicitly, which is interesting, and I think they've also (unintentionally; there's no discussion of the homoeroticism here) put their finger on why Ewen is such a satisfying character to slash, which is perhaps slightly less obvious than why Keith is. They also speculate about why Broster's popularity has waned and why she might appeal less to modern readers.
The History of British Women’s Writing, 1920-1945 (Volume Eight), edited by Maroula Joannou, in a chapter by Diana Wallace, 2013. Largely a less detailed restatement of Wallace's ideas from her own book (see above), but a nice succinct statement (and available on archive.org!).
I've included links to archived copies wherever possible—most of these require an archive.org account in order to read more than a couple of pages. Let me know if you can't access these for any reason—I've got typed-up copies of all the pieces and will happily send any of them to you.
The Horn Book Magazine, volume 5, number 1, February 1929, article by Eleanor Whitney. Short article about Broster, and—most interestingly—a further contribution from Broster herself! This is the only writing of Broster's about her own life and work that I've found, and contains lots of interesting detail both biographical and about her writing process. The ultimate source of this letter is rather mysterious; the Horn Book Magazine claims it was written for them, but a slightly different version of the same letter appears in The Junior Book of Authors in both 1934 and 1951—the latter claiming that it was written for them shortly before her death, which certainly isn't true. Also the Dictionary of Literary Biography (see below) quotes bits from it alongside other quotes not in it, describing the source as an 'undated typescript for a speech' in St Hilda's College archives, and Diana Wallace (also see below) cites something that sounds like this piece as a source for information that isn't in it; which suggests there's yet another, longer and more detailed, version of the same piece in the college archives.
Mrs Beer's House by Patricia Beer, 1968; pp. 115-118, 185-186. This is one of several autobiographies to mention Flight of the Heron as an important part of the young author's reading, but it's by far the most detailed, and it's generally delightful. Beer is the earliest instance I've found of someone definitely recognising the homoeroticism of Ewen/Keith; she also shares our mystification over the short word beginning with S which BPC was going to call Ewen in that one scene, and includes some of her teenage poetry inspired by Loch na h-Iolaire!
Twentieth Century Romance and Gothic Writers, edited by James Vinson and D. L. Kirkpatrick, entry written by E. F. Bleiler, 1982. Not very detailed, but a nice overview, and I agree with their assessment of the ending of The Dark Mile.
Dictionary of Literary Biography, Volume 160: British Children’s Writers, 1914-1960, edited by Donald R. Hettinga and Gary D. Schmit, entry written by Ruth Waterhouse, 1996. This is one of the longest and most thorough sources, covering all Broster's books (except Child Royal, which is inexplicably skipped) and her life in some detail; it's well-researched, using material from St Hilda's College archives, and contains some thoughtful discussion of the books and their themes. Waterhouse says some interesting things about Flight of the Heron, treats Almond, Wild Almond more sympathetically than I'm inclined to—making some decent points, though I can't quite agree—and, somewhat oddly, views both Ships in the Bay! and World Under Snow as entirely comic/parodic works.
The Cambridge Guide to Women's Writing in English, edited by Germaine Greer and Elaine Showalter, entry written by Valerie Purton, 1999. I'm not impressed with this piece, which contains multiple minor factual errors and rather badly misrepresents the themes of Flight of the Heron (yes, gentlemanly values are important, but the one crucial point where Keith despises 'womanishness' in himself it's clearly an incorrect judgement of something good, and I don’t think Ewen frames his gentlemanly values in a particularly misogynistic way. And the characterisation is so not simplistic!).
Contemporary Authors, edited by Scot Peacock, 2003. The writer of this entry gets an impressive amount about the books wrong (did you know that the main characters of The Wounded Name are on opposite sides of the Napoleonic Wars—we're not informed whether Aymar or Laurent is the Bonapartist, sadly—or that Raoul des Sablières is actually an Englishman???) and has relied heavily on the Dictionary of Literary Biography for the rest, but also includes some biographical details that haven't turned up anywhere else, and frustratingly without citing any primary sources.
The Woman's Historical Novel by Diana Wallace, 2005, pp. 7, 29, 33-34. While limited in scope, this is one of the most fascinating sources I know of, and it's therefore particularly frustrating that it doesn't seem to be archived online anywhere. Message me if you'd like a copy of the relevant passages! Wallace discusses the homoeroticism of The Flight of the Heron in the broader context of women's writing about relationships between men and oblique representations of sexuality; she argues that the fantasy element in the book, the prophecy foretelling Ewen and Keith's meetings, gives this forbidden, repressed attraction an opportunity for subtextual expression.
Out of the Attic: Some Neglected Children’s Authors of the Twentieth Century, edited by Pat Pinsent, entry by Chris Clark, 2006. This is the longest and most scholarly discussion of Broster's work I've found, and contains a lot of food for thought. It compares Broster's writing to that of another historical novelist, Cynthia Harnett (I only skimmed the parts about Harnett, not having read any of her books, but I may have to see if I can find them at some point!), and analyses the sentence-level craft of The Flight of the Heron and The Gleam in the North in great detail. Clark articulates a lot of things about Broster's writing style and characterisation that I was vaguely aware of but hadn't laid out for myself this explicitly, which is interesting, and I think they've also (unintentionally; there's no discussion of the homoeroticism here) put their finger on why Ewen is such a satisfying character to slash, which is perhaps slightly less obvious than why Keith is. They also speculate about why Broster's popularity has waned and why she might appeal less to modern readers.
The History of British Women’s Writing, 1920-1945 (Volume Eight), edited by Maroula Joannou, in a chapter by Diana Wallace, 2013. Largely a less detailed restatement of Wallace's ideas from her own book (see above), but a nice succinct statement (and available on archive.org!).