Once you get past the initial hump of learning how to program, you need to learn how to program games.
Even if you've been in the business for many years, you may have ended up seeing one, two, or if you're lucky, three different game engines from the inside.
However, this book ties together various nuggets from literally dozens of different game engines, written by the engineers who worked on those engines.
Some gems will not work well with your current game, others may be just what you need -- however, the real value comes from the education and the illumination of different approaches to solving common problems in game development.
Reading a book like this is an invaluable way to grow as a game engine programmer.
Disclaimer: I wrote one of the gems, so if you buy this book, I will earn about three cents. That's not why I think you should buy this book -- I think you should buy this book because it will truly expand your game development horizons! That's also why I contributed in the first place, and I think Eric has done a great job of editing and putting it all together.