Practical UNIX & Internet Security, 3rd edition - Focusing on the four most popular Unix variants today--Solaris, Mac OS X, Linux, and FreeBSD. Includes table of contents, errata, and sample chapter - http://www.oreilly.com/catalog/puis3/
Computer Security Basics - Describes complicated concepts like trusted systems, encryption, and mandatory access control in simple terms. - http://www.oreilly.com/catalog/csb/