Volcanoes! Mountains of Fire, by Eric Arnold, illustrated Doug Knutson.
This book is a decent introduction to volcanoes. The text is written at a second grade level, and the illustrations are all drawings. It gives examples of a couple of different volcanoes, but really doesn't delve into the properties of the Earth's crust or how volcanoes are formed.
If you want to, er, dig a little deeper, check out a volcano book from the adult section of the library.