Having just bought my first bike in years, I was looking for a book to give me a summary of mountain biking in one simple package. I bought this book, though nearly didn't as the other review here was quite negative and I was worried it would mainly be a handful of photos and very little information.
Fortunatley, it turned out to be a cracker and gave all I needed to learn the lingo, be familiar with different type of bikes, competitions, gears, brakes, etc. It goes into more detail you would think, discussing even the newer composite frames and magic alloys dismissing some of them as marketing bull! It's wrote in the style of a magazine, so will also tell you what products to avoid if the author thinks they're not worth it.
I bought several other bike books at the time, but this seems to cover all of the topics in a simple package. The great photos and simple explanations mean it actually was more informative than some of the heavier material. The maintenance section is a bit brief, but perfect if you're a fairly casual rider or take your bike to the shop for big stuff - it's not meant to be a maintenance book (Haynes do another much more detailed bike book for this, called "The Bike Book" no less!).
It's a perfect summary of the entire spectrum of mountain biking and will make a lot of sense of all the stuff you see in the shops, the banter you see on forums and even make sure you ride it properly.