This is a collection of things people have written—in literary dictionaries, scholarly books, magazines and various other types of sources—about The Flight of the Heron and its author. I've included links to archived copies wherever possible—most of these require an Internet Archive account to view more than a couple of pages, but should then be fully accessible.
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. See also Typescript 3 in Writings of D. K. Broster, which is probably Broster’s original copy of this letter, now held in St Hilda’s College Archive. Note that while the Horn Book Magazine claim the letter was written for them, and its date corresponds with that, 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 Broster’s death, which is certainly not true.
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. 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, 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!).