Support local bookstores and try to buy your books from places like this. Great vibe here, the place is pretty small so don't expect to find every book you might be looking for.
In addition to books they have a small selection of vinyl records with bunches of rare titles.
Caliban is the home of Pittsburgh literary magazine "The New Yinzer" so check out their website and support Pittsburgh's local writers.
The reason I am giving 4 stars instead of 5 is because they don't have a great selection of paperback fiction.