http://ecx.images-amazon.com/images/I/417HWkYAEDL._SL500_AA240_.jpg

isbn:0967584426

PolicingOnlineGames is a book by PeterWayner that discusses the use of cryptographic mechanisms to support online gaming.  E.g. rolling dice, playing cards, trading items, exchanging money, resisting counterfeits, scavenger hunts, spy vs. spy dungeon crawls, etc.  

As part of explaining these, PeterWayner does an excellent job explaining cryptographic primitives and how they are put together in practical settings... all in a tiny, easily absorbed volume of 116 pages.  I'd give it a hearty recommendation.