There's no real theme to this book apart from a loosely conected batch of useful stuff for the players.
Each of the different tribes of werewolves is detailed further. We find out the legends that each tribes has on it's origins, and some information on how they interact with the world of humans.
There's also an interesting part on werewolf society, and how they work together.
Additionally, there's a lot of miscellaneious information, such as new skills, new totems, magic itens and Gifts (special powers, learned from spirits) and an optional system for merits and floaws, an optional system of little extras & quirks that can add colour to a character.
By far the most interesting section is the one detailing non-werewolf shapeshifters. A long time agho, the werewolves waged a senseless war on all of them, making the survivors rare, reclusive and distrusting. Each is gone into in a fair amount of detail, though White Wolf are presently putting out a seires of books that deals with each species individually. The most interesting are the Mokole (were-reptiles) who have in addition to their animal and human forms, can call on the form of an ancient 'reptile king' such as a sea-serpent, dragon or dinosaur.
There are also details on the three lost tribes of werewolves, the Crotoan, the third Native American tribe who sacrificed themselves to drive back a manifestation of the Wyrm itself, the White Howlers who were corrupted and devolved in prehistoric times, and the Bunyip, an Australian clan who were wiped out by their fellow werewolves.
The PHB does a good job of adding depth to the werewolves. Sure, they're fearsomely powerful killing machines, but attention is drawn the the terrible consequences of past abuses of their power.
In summary, this book is a worthy addition to anyone looking to add some depths or something unusual to their characters.