Does anybody know where to find a shell's collecter's book that a tennager could understand?

If you are in the United States, a good, inexpensive starting book is Seashells of North America by R. Tucker Abbott. It's one of the Golden Field Guide series. It has a good amount of general information in the first 50 pages, written in non-technical style, followed by excellent color pictures (artwork, not photos, but very realistic) of several hundred species. It's available in hard cover or paperback.

If you are not in North America, mention your location and I'll try to recommend something.

If you can find a copy of "Collectors Guide to Seashells of the World by Eisenberg it contains a lot of shells in clear color pictures. A couple are up on ebay now. It was reprinted and in this case I think to the good because the reprint is larger in size so the pictures are bigger. It's not overly technical and a teenager should be able to handle it.