Quite a lot of the general principles and guidance in the book is still vaid. Quite a lot (25%) of the detail has changed.
Since the book was written, the developer docs (http://docs.moodle.org/dev/Main_Page) have been improved a lot.
Also remember that Moodle comes from a social constructivist philosophy (http://docs.moodle.org/22/en/Philosophy). Embrace that and learn by doing, and learn from your peers.