Animorphs
by K. A. Applegate
Not only was this series very well written and intriguing for a concept that has been played out many times(small band of heroes must face off against a giant horde of enemies), it featured great cover/interior book cover illustrations. The storyline was about 5 teenagers who were given powers to morph into any animal they have touched, and use that to defeat the slug-like mind controlling yeerks. The thing about the art was that it showed the people turning into the animals/aliens, but it looked so realistic. It would also feature a interior drawing that used the head of the body from the cover in a situation that happend in the book. Also rendered quite nicely.
Not only that. You’d get a little picture at the bottom of the person morphing, by flipping the book flip book style. It isn’t really all that much, but it makes the book that much better than most books with just a cover illustration.