I've read both this and the ExtremeTech "Hacking Google Maps," and both have their place, but I found this one to be much more valuable.
It doesn't contain any introductory information to the API, which some may be looking for, but it's not something that I would expect to see in a Hacks book anyway. With the v2 release of the API in April 2006, Google's own documentation has gotten a lot better, and it's the best place to go for a tutorial.
The book contains a great collection of hacks from all perspectives: users looking to get the most out of Google Maps, power users looking to push the functionality in new ways, and developers using the API. There's also a good overview of some popular mashups from the mashups' developers perspectives, and extensions to the API (e.g. TLabel, TPhoto).
Because there are so many contributors, you'll notice a change in tone and writing style throughout the book; and different coding practices are easy to spot, e.g. one code sample might use a standard, documented JS call, while another uses a homegrown function to accomplish the same thing. Another drawback is that the book is written to v1 of the API, which was replaced months ago, although is backwards compatible. This doesn't affect readability or value, but several of the undocumented features in the book are in fact documented in v2!
Great book, filled with useful information.