Sophos has provided their 2012 Threatsaurus free for download. It is a glossary of common terms in data security presented at a casual level that makes it easy to understand and fascinating to learn. The 131-page book goes through security terms, security software and hardware, safety tips, and even a history of malware.
Best of all, the PDF can be obtained free of charge as a direct download without having to register, give your email address, or listen to one sales pitch.
Download the 2012 Threatsaurus from Sophos.