I bought this book to get a better idea of how to safely interface my own circuits to the Lego Mindstorms NXT. In my opinion, you can't find a better book for this purpose. "Extreme NXT" contains the pinouts and proper usage of the NXT motor and sensor ports. It also provides a large number of the most useful homebrew sensor implementations, right down to parts lists containing part numbers for common component suppliers like Digikey and Radio Shack.
My background is in engineering, with an emphasis in control systems. The first book I read in the field of hobbiest robotics was "Mobile Robots: Inspiration to Implementation" (Jones and Flynn). In those days, the 6811's were king for these sorts of projects. I spent many a happy day with my MIT Handyboard (a simple 6811 robot controller board). Things have come a long way. Nowadays NXT is available, and this book is the modern day equivalent to that excellent text for those who want to dig in and find out what the NXT is capable of. The book is written in a format that is easy to understand for the novice, but also supplies plenty of content to allow the more advanced user to be able to push the NXT envelope. The organization is superb, commonly used charts and tables are easily found. I have been finding myself referring most often to the pinout tables for the NXT connectors, and these are very conveniently found towards the front of the book.
Chapters are broke down into sensor and output types, starting with the more basic and easily contructed passive sensors up to more advanced and expandable I2C sensors. Motor control, h-bridges, etc are also covered in detail. Both the construction of the circuits, and the NXT code to run them, are covered. Code is provided in it's most basic form first, NXT-G, which every NXT user will be familiar with. Examples proceed to more complex compilers, such as NBC, for sensors or circuits that are better suited to those implementations.
The appendix provides complete source code listings, useful websites and links categorized by chapter. There is also a great tutorial on prototyping with breadboards and perfboard for the uninitiated. With this book, any skill level in electronics should be enough to do some cool things with the NXT kit.
This book will become a very useful addition to any hobby robotics library. For me, it will be the goto book for designing circuits to safely interface to the Lego NXT. Highly recommended!