Cryptography for Dummies is a really good introduction to cryptography.

Cryptography is one of the most intimidating aspects of computer security, conjuring up, as it does, such concepts as hash functions and public-key infrastructures. For the average user who wants to know about cryptography without gaining the proficiency of a cryptographer, Cryptography for Dummies is the perfect introduction. 

The book details the core elements of cryptography that the average user needs to understand, leaving the theoretical topics to more long-winded texts. Emphasis is put on simplicity and straightforwardness, with as little gibberish as possible. Screen shots and illustrations are used effectively without being condescending or insulting. 

As the book progresses, the chapters plumb more detail. Those wishing just a quick introduction should stop after part one. For the more ambitious, sections on public-key infrastructures, secured sockets layer, authentication systems, and virtual private networks lie ahead.