There's a second-hand book sale scheduled to take place at the Neilson Hays Library on Saturday June 19 and Sunday June 20, 2010. The book sale opens at 9.30 am and closes at 4.00 pm. All book sale proceeds will go to the library.
More about the library and how to get there available here.
BOOK appears to be the name given to a small independent bookstore selling used books in Bangkok. This bookstore is located in a small sub-soi near to the entrance of Sukhumvit Soi 24, less than two minutes' walk from BTS Phrom Phong and the Emporium Shopping Complex. It is also surrounded by massage shops and restaurants catering mostly to Japanese and Korean businessmen and their families living in the vicinity.
Angel Tomorrow imports used books for sale in its small warehouse located on Sukhumvit 105 (Soi Lasalle). It is run as a part-time business by a husband and wife team who previously owned a second-hand bookstore at the Seri Center, near Seacon Square. Books are imported from the US several pallets at a time and then unpacked and sorted in the warehouse.
If I were a 20-something office girl working in Sathorn, I’d probably list Double A Book Tower as one of my top lunchtime destinations, followed closely by the som-tam shop and clothing stalls selling bargain tops for 80 baht a piece, or two for 150 baht.

Skoob! is a small bookstore and cafe that sells new and used books in the Thonglor area of Bangkok. It gets its unusual name from the word BOOKS written backwards. The store sells English and Japanese titles.
Bangkok Rare Books is located in Amarin Plaza, a short walk from BTS Chitlom Station and Central World Plaza.
Sun Books is a used bookstore that buys and sells Japanese books and magazines. The bookstore is located next to Phrom Phong BTS station, opposite the Emporium Shopping Center.

Dasa Book Cafe (Thai: ร้านหนังสือดาสะ) pronounced "daa-sa" with a hard "sa" as in "sack", is an independent bookstore and café selling secondhand books. It has more than 13,000 used books in English and other languages, and claims to be Bangkok's best-stocked secondhand bookstore.
