1. CU Book has lots of academic books. There are also lots of international edition books that is much more cheaper than books in the United States. 2. Kinokunika book store has lots of fiction and non fiction books. There are some of the Thai language books for foreigners that written in English. In additional, There are many Thai - English dictionaries. 1 and 2 located at Siam area. You can get there by BTS - Siam station. 3. You can find tons of used books at Jatajak weekend market. You can get there by MRT (subway) exit the train at Kampangphet station. 4. Asia book is also located at Siam Paragon (Siam area). It has lots of Asian related books. I suggest you to visit this book store to learn about Asian culture.

What about CU book center? I think there is a good book store. It was located in Chularongkorn University.

If you live downtown you can go to Kinokuniya at Siam Paragon on the 3rd Floor. There, you can find a lot of books, not only Thai, but also English, Chinese, and Japanese books. And they have the computer for you to find a book you want. It's very big so you might have to take your time to look around.