If you're looking for hard to find or out of print mystery novels as I am in NYC, you may want to try this bookstore -- Murder Ink.
They have quite a good selection. Plenty of rare finds and early edition novels. In terms of their quality of goods, I rate them highly. And in terms of their service, it was exceptionally good --- I have given them calls to see what was in stock and they have been helpful to locate my items and reserve them for me. Really friendly help.