The Documents in the Case, first published in 1930, is in many ways an unusual book. It's the only novel that Dorothy L. Sayers wrote in collaboration - with Robert Eustace. It's her only novel which does not feature Lord Peter Wimsey. And it's a novel that offers not only an interesting and unorthodox "howdunit" mystery, but also a fictionalised version of the Thompson-Bywaters case that fascinated a good many Golden Age novelists, not least Anthony Berkeley and E. M. Delafield.
Sayers is not by any stretch of the imagination a forgotten writer, but this is the one novel of hers that seems to me to have been generally under-valued. Perhaps it's the absence of Wimsey that accounts for the generally lukewarm critical reaction over the years (though I should add that a number of good judges have also praised the book.) Each time I read it, I find my appreciation of Sayers' skill increasing, even though, in the immediate aftermath of completing the book, she felt that she had failed to do justice to the clever idea at its heart.
In telling the story, she borrowed from Wilkie Collins' The Moonstone. Collins was a writer she greatly admired, and she was at the time she wrote the novel working on a biography of him that she never managed to complete. The events are seen from the point of view of characters in the story, and told in the form of letters. It's a very good device, when used well, and Sayers captures the different voices of the characters splendidly.
It's in the later part of the story, where the scientific material central to the plot is debated, that the narrative flags somewhat. Eustace contributed the scientific ingenuity here, as he did to stories by other writers such as L.T. Meade and Edgar Jepson, but Sayers did the writing. The method she chooses for conveying this material results in a rather abrupt ending, The problem with this section of the book is one of story structure, and I sense that she rushed those final pages, when with a slightly different approach she might have produced a book that would have been more widely acclaimed as an innovative masterpiece. Nevertheless, it's a novel of considerable interest and distinction, and if you aren't familiar with it, then it's definitely worth a read. There's a fascinating chapter about how the book was written, incidentally, in the late Barbara Reynolds' excellent biography of Sayers.