I have to be honest, I completely love this book. Without this book, picking deities basically consists of running your finger down the list of gods in the cleric's list, and finding someone who matches your alignment or vague ideals. At best, without this book, you could pick up the Campaign Setting book for $50. Not only does this book save you money, it adds an incredible amount of depth to the character creation process.
For the people who prefer the table approach, the inside cover offers a tabled list of 61 deities, both major and minor. They are laid out in the comprehensive setup we are all used to in the Core Rulebook. There being so many of them, they are really crammed in there, but the text is decently sized and easy to read.
The book itself starts off with a very brief explanation of how the deities got where they are now, and a small setup of the mythology. It also explains how one can become a god, for later reference.
Immediately, on page 4, the author goes into information about the 20 major deities. Rather than a few paragraphs, the book offers two full pages of text, first describing the deity and how they rose to power, then the temples and devotees likely to associate with that particular entity. Festivals, dress, and customs are detailed as well. Included is a fully detailed and beautifully drawn representation of the god or goddess, in their favored form. For each entry, a bonus spell is added for people capable of divine casting.
I cannot overstate how spectacular this section is. Admittedly, I am a deity-heavy player and GM. This book adds a vast wealth of information for both players and GMs, ready for implementation as soon as one opens the book.
The major gods are followed by several pages of minor gods, in case the 20 core are not enough. Many people prefer not to pick the most popular choices, and make a more elaborate and unique character, and this offers choices on doing exactly that. As noted by the author, these are mostly the same as presented in the Pathfinder Campaign Setting book, and only reprinted here for reference in a less expensive book.
After this is a section about magical items, some of which are fairly interesting. An example item is the Robe of the Master of Masters, which changes color according to the will of the wearer. It grants bonuses to Perform (dance), and has a built-in switch which grants Haste for 1 round. It also allows for some minor healing. This sections presents 38 new magical items in total. It also presents 4 artifact items.
What this book is short on, is clearly stated rules. I only add this part because this is almost purely a roleplay book. Many people (read: power gamers) would consider this a "fluff" book, with nothing but a bunch of monotonous writing about all that roleplaying nonsense. For anyone serious about making a character with personality, this book is incredibly essential.
All said, only the back half of the final page is unusable, as it is an advertisement for another book. Typical books cram ads in at the back, but this one forgoes that. Starting from the inside cover, even before the first page, nearly every inch of this book is useful.
This book should be considered mandatory for PFS play, or for anyone who takes their character's spiritual beliefs seriously. I can't imagine making any divine character, or even a pious martial character, without the use of this book. For the $17.99, this book is absolutely worth every penny. I can't imagine anyone being disappointed with Gods and Magic.