My Computer Security Textbooks
There are two textbooks. The original one is called Computer Security: Art and Science and is now in its second edition. An alternate version, called Introduction to Computer Security, is much shorter, and "omits much of the mathematical formalism, making it more accessible for professionals and students who have a less formal mathematical background, or for readers with a more practical than theoretical interest" (from the blurb on the back cover).
Click on the picture below to go to the right web page. Or visit them both!