Royally Wrong is a picture book adaptation of a scene from Shrek the Third; specifically, it's the lead-up to Shrek's quest to fetch King Arthur. The subplot about Fiona's pregnancy, which is present in the movie's version of this scene, is cut entirely.
Frankly, there's no real reason for this book. If your child desperately needs reading comprehension and basic language skills reinforced and also loves the third Shrek movie, perhaps it'll be a good book for you.
Otherwise, it's just another piece of rather pointless kiddie flick merchandise. *shrug*