This book doesn't tell you anything that isn't in the manual, but the manual is a dry old thing of about 225 pages! The information about getting best use out of what is a very complex camera is presented in a way that makes it so you enjoy reading it and subsequently find absorbing the information far easier than just reading the manual. Don't get me wrong - you still need the manual, but the manual is quite hard to read from cover to cover.
If you are a digital photographer upgrading from a recent Nikon digital camera to the D5000, you would probably be wasting your money. If, like me you arrive from an older camera (Olympus E10), the two cameras are worlds apart and you will benefit greatly from the easy-going format of this book.
I found that reading this first meant that the manual then became useful to carry and refer to.