By far the best serious book that presents the basic tenets and philosophy of Wicca as religion and isn't horribly watered down is Vivianne Crowley's "Wicca, a comprehensive guide to the old religion in the modern world" . Another good choice is Ellen Cannon Reed's "Heart of Wicca", which gets into the concepts of initiation, how it works and what it means. I highly recommend both for anyone with a serious interest.