Bone Handbook by Jeff Smith
The Bone Handbook is a collection of extras about the Bone saga. It includes a history of the Valley where all the action takes place (basically a summary of the whole story), character biographies, recipes, interviews with the writers and the colorist, and a couple of bonus stories. It's the graphic novel equivalent of that second DVD full of extra materials you watch because you really love the movie. I found it enjoyable and informative and recommend it to Bone fans--but you need to finish the graphic novels first or else you'll read spoilers!