Ambassadors for Christ Bookstore | A good source for books in Chinese. The site has English categories into which the inventory has been organized, though many of the titles and descriptions are available only in Chinese.
Tien Dao Christian Media Association | Another site that sells books and tracts. The site itself is mostly in Chinese, but the search function allows you to enter English titles that may have been translated into Chinese.
Asian Book One | Still another good site for books. There’s overlap with the other booksellers listed here, but also unique titles as well.