Book Gallery might have one of the best overall collections I've ever found in a used bookshop. And I've seen a LOT of them.
My experience here was mainly with books in the science, technology, and transportation genre. The selection was so broad and deep, I'd imagine that it must be incredible in the more mainstream categories.
Prices were fair and reasonable, given what I knew about the titles I was finding.
I haven't yet checked out the Phoenix location, but the Mesa proprietor says the selection there is even wider.
I almost can't imagine it could be.
Hours are 10am - 6pm Mon-Sat, by the way.