I nominate
The Case of the Silent Partner by Erle Stanley Gardner. Published in 1940, it wasn't the first Perry Mason book (that would be
The Case of the Velvet Claws in 1933), but all the books can be read as stand-alone mysteries. If we need a first, then let it be called the first of Garner's Lt. Tragg books, for this is the book that introduced Arthur Tragg to the reading public. The beauty of these earlier works is that we see a much rougher around the edges Mason, someone who took a lot of chances, wasn't afraid to use his fists when the occasion called for it, and even, in the pursuit of justice, said, "Legality be damned."
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