I'll preface this review by saying that I've not been particularly sympathetic to Werewolf in the past. But Werewolf 20th Anniversary Edition has been enough to intrigue me, and soon I'll be playing in a short Werewolf game to see if there's anything for me in it.
This book is very thorough, if not quite as massive as the book in the Mage series. It covers who the Garou are, their mythology, cosmology and society effectively right up front, before going into who an individual Garou is.
The two most interesting chapters, though, are the Spirit World, which explains how the Umbra is laid out and how Werewolves interact with it. The next is The Enemy, which cover Black Spiral Dancers (to more-or-less the same level of detail as the Gaian tribes), Fomori and other enemies. An honorable mention goes to the end of the chapter on Allies for giving spreads for the Lost Tribes and the Changing Breeds (who get more detail in the Changing Breeds book).
Overall, the book is solid, but it does have problems. There are a few layout errors (bolding the wrong text, etc), and it falls apart a bit when crossover is mentioned (my favorite page involves them trying to refer to the Zigg'rauglurr from Mage). And of course, a core problem for Werewolf: the game itself seems to be fairly pro-eugenics, and there's not really any way to separate that out.
[4 of 5 Stars!]