The Nikon D90 comes with a fat manual that tells you absolutely everything but explains absolutely nothing.
Ben Long's Companion book takes a far more practical approach, moving in easy stages from automatic point-and-shoot use of the camera to topics so esoteric that I cannot believe even professional photographers really need them.
It covers most features of the camera, and includes practical examples of how to use it in its various modes---and explains what you can gain from learning to control the camera yourself instead of leaving it in Auto.
As a bonus, there are chapters on how a DSLR works, the technical details of photography (for example the relationships between aperture, shutter speed and "film" speed), plus practical help with how to take good photos.
Written in a clear and easy-to-understand way, this book is definitely recommended, either for a beginning photographer or someone new to the D90.
I would even recommend it for anyone who just wants to know more about digital photography in general.
So why not five stars? Well the book is let down by a few spelling mistakes, missing words, and one strange contradiction. Is the camera 12 MegaPixels or 10? The author seems to have two views on this subject.
Maybe it's the way you count them.