E-A, from your post, not sure what books you are looking for. There are quite a few places in Chulia St, for example, that have lots of second-hand paperbacks. Most of those places operate a straight swap system if that's what you are looking for.
However, at the back of the Chowrasta Market, on the first floor, is a place that has a very large selection of old books and magazines. Huge piles in the alleyways and on the shelves covering pretty much all subjects from reference to fiction.
If you need more info, please PM me.