I don't know if they're available elsewhere or not, but the outstanding four books in Margery Allingham's "Campion" crime series have just (yesterday) been released as ebooks, meaning that the whole series is now available. One of the new books is the first one in the series, "The Crime at Black Dudley", which is a bargain at £1.99.
I've been waiting for years to read this series, because I always prefer to read series in order, and the fact that the first one was missing was irritating
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Allingham, along with Agatha Christie and Ngaio Marsh, is often called one of the "Queens of British detective fiction", and her "Campion" series is one of the classics of the so-called "Golden Age" of British detective fiction.