This is an overall excellent title, and a “must-have” as far as third-party supplements go!
While it is primarily billed as a collection of deities to add to your game world, it is actually so much more. Perhaps the real meat of the book is the expanded system for Icon Relationship rolls. What is probably 13th Age’s defining—and lest understood—mechanic is thoroughly explored and explained within "Gods and Icons". It doesn’t change the core mechanic so much as quantify exactly what it is capable of. This is great for defining the level of rewards and setbacks that relationship rolls should entail, without players perceiving the system as vague or nebulous as it sometimes was before.
It also offers a nice balance of content for players and game masters alike, so no one is left out in the cold. There are three fully fleshed out pantheons at your disposal and an entirely new set of Icons. Each has enough fluff to be used exactly as presented, but there are more than enough individual ideas to inspire any GM should they wish to dig deeper. There are also a handful of fantastical locations, complete with story hooks. Players have access to several new (and super unique) races, additional feats for core classes, and a huge list of fun and funky magic items.
If you enjoy 13th Age, this title has more than enough content to satisfy. Even if you don’t use everything as presented, you’ll find something to inspire your own take on things. It isn’t strictly limited to 13th Age either, as the beginning of the book includes suggestions on how to utilize concepts and ideas in other systems.