Dinosaurs: The Most Complete, Up-to-date Encyclopedia for Dinosaur Lovers of All Ages, by Dr. Thomas R. Holtz, Jr., and illustrated by Luis V. Rey (Random House 2007). This is a terrific book and, as the title suggests, both comprehensive and appropriate for ages middle grade and up. Particularly novel is the complete dinosaur genus list in the back.
Paleontologist Holtz provides excellent explanations of, and descriptions of, all of the dinosaur families. He also includes extensive discussions on the science of paleontology, cladistics, geology, etc. Illustrator Rey adds color and verve to the account, providing radical and eye-catching interpretations of dinosaur soft tissues and feathers, reminding us that we really have no idea what dinosaurs may have looked like in the flesh.
Definitely a must-read for dinosaur aficionados of all ages.
Dr. Holtz has a web page full of updates and supplemental material here.