Generally speaking, English books in Taiwan are expensive and don't always cater to the needs of expat families. There are two great places to shop, though:
Cherry Valley Bookstore : Operating in Tienmu near Taipei American School as well as online, Cherry Valley Books is run by an expat dad and Taiwanese mom. They endeavour to supply reasonably priced books, including periodic sales of secondhand books they bring in from abroad.
Book Depository : With free shipping offered worldwide, this is a great option for buying books online. It works out cheaper than amazon, and means you don't have pressure to buy more than that one book you need. Book Depository is growing fast, and they have a really nice selection of books if you are looking to supplement your child's English reading and education.