I can't recommend any books personally, but when I started learning German (also at 13 years of age), I too wanted to do something extra in my own time - as the pace in school can be rather slow, so I can relate to how he feels.
I found the Deutsche Welle website to be useful. It has plenty of fun learning resources - for a variety of levels (online lessons and games etc):
http://www.dw-world.de/dw/0,,2547,00.html
Hope that helps