I picked this book up for my 4 year old who loves to draw monsters and other creatures. We've had other step by step drawing guides and I found that my son can follow them pretty well at his age. What I like about this book is that many of the characters start out with the same basic shapes. I think this works well with reinforcing shape recognition and getting the child to think about different ways to manipulate shapes to create new characters. The steps are easy to follow.
My son's only complaint is that the monsters weren't creepy or scary enough. The monsters all wore happy or silly expressions. I think they're cute. But if you're looking to draw some creepier monsters check out Monsters!: Draw Your Own Mutants, Freaks