According to many of the best-read in crime, P. J. Wolfson's Bodies Are Dust from 1931 is an overlooked classic. It's very hard to find. While I've almost never seen it online for less than $100 (usually more like $200), it showed up recently for only $30, so I grabbed it.
This edition is a Lion reprint from the early 50's. It cost all of a quarter when new.
I am not a book collector by any means, but I'm going to have to work up some nerve to take off the shrink-wrap. The card tucked inside on the back dates it as having been untouched for about thirty years.