If you are feeling lazy or have a specific book you are looking for, online bookstores are a great resource. Do bear in mind that a handling fee is charged on top of tax for online book sales. On the plus side, shipping is sometimes free.
Fishpond.com.sg - Online megamall that has a book section. I think the prices are a little higher than average.
Acmamall.com.sg - Smaller multi-product website that carries books. Good selection at reasonable prices.
NoQstore.asia - Large selection of titles and a small selection of stationery and accessories. Currently having Christmas sales that are up to 50% off or S$10 per book specials.
Kinokuniya.com.sg - Has an actual store at Ngee Ann City. Some of the books are available for sale online.
Bookfishing.com.sg - Buy and sell comic books, assessment papers and textbooks.
Skoob.com.sg - A new online bookstore that has free books, not sure what that is about.
ilovebooks.com.sg - English and Chinese titles available at this online bookstore
bookdepository.co.uk - UK based online bookstore that ships free worldwide!