Living near the Microsoft campus, we are well served by the University Bookstore (the one in Seattle, not in Bellevue). The Seattle public library is quite famous too, although mainly for its architecture.
The best I've seen in this country is the University store in Menlo Park. They probably have more than 500 books just on number theory alone. The other academic store in Palo Alto is pretty good too. At the opposite end, I was once walking in a very large, beautiful bookstore in Los Angeles, one block away from our VC headquarters. At some point, I asked an attendant where the math/stat section was. She paused for 20 seconds, then looked at me as if I was crazy, possibly dangerous, or coming from a different planet.
Anyway, my wife claims I buy books depending exclusively on colour, size and how they fit in my bookcase. She claims I get books organized by colour and size (and possibly other obscure criteria), but that the contents does not matter.
I also frequently visit half price book in Bellevue, Washington, and once purchased (in 2008) some ACM proceedings (1998) for $20 while the 1999 edition on the same shelve was for sale for $100. And once purchased 30 volumes or so of the Encyclopedia Britannica for $20 (entire collection at about 0.1 cent per page), less than the paper value. At the other extreme, Handbook of Data Visualization (Springer Handbooks of Computational Statistics) at $319 for 900 pages might be the most expensive book, at 30 cents per page.