Sometimes the question of what counts as a "forgotten book" is open to debate. One might use that term to refer to books by living authors that have been out of print for a long time, for instance, But I think there can't be any doubt that Roy Horniman's 1928 novel The Viper qualifies for the description of "forgotten". I've never come across any discussion of it anywhere.
If you do an internet search for the book, you may however find that I mentioned it here rather more than seven years ago. At the time, I'd just purchased a copy, though I confess that I've only just got around to reading it. I mentioned The Viper in the context of a discussion about Roy Horniman's most renowned book, Israel Rank. Though "renowned" is a relative term here; Israel Rank is a fascinating novel, but compared to the film version, that wonderful classic Kind Hearts and Coronets, it is still somewhat undeservedly neglected.
Why did it take me so long to read the book? One reason is that I was deterred by the opening paragraph, one of the dullest one could imagine, which is about train times from Weston-super-Mare to London. It makes the more tedious passages of John Rhode/Miles Burton seem as sparkling as Simon Brett or Colin Watson. But soon the story warms up, and it becomes clear that one of the travellers on that train is a sociopathic monster.
This fellow is a con man who is prepared to stop at nothing to get his way. He has his eye on a fortune, and he is ready to kill the innocent in order to lay his hands on it. In this respect, at least, there is some overlap with the famous storyline of Israel Rank. Perhaps Horniman was aiming for a comparable effect. But I can't claim that this book is a classic, although there are some nice ironic touches. Instead, what we have is a thriller that is, thank goodness, livelier than that opening paragraph suggested. Not a masterpiece, by any means, but quaintly readable.