I recently bought some really interesting books at a small bookstore, USA BOOKS, 100 Pham Viet Chanh, district 1, HCMC. It's a really nice space, small cute shop, cheerful employees and very comfortable arm-chairs for readers (you don't have to buy books, just sit there and enjoy). I usually spent my weekend afternoons there, very enjoyable.
Besides, their order service is cool too. They can order any book online for us and deliver within 3 weeks. The fee is also really competitive, cheaper than any other bookstore in HCMC.
Come there and check it yourself. Whether you need a book or not, this is still a good place to come.