Although I studied the subject formally in some semester many years ago, I was always interested in this subject and I found very helpful to develop a mindset on what constitutes computer security and defense at various levels and sizes from a personal machine to a corporate network early on, using various books besides my notes. I found interesting, books like "Maximum Security" by
Anonymous, that is mostly a practical guide of tools and techniques for protecting a network, the series of "Hacking Exposed" by
McClure - Scambray - Kurtz - a fairly recent edition here
https://www.amazon.com/dp/0071780289/?tag=pfamazon01-20, that is about practical advice, techniques and tools of defense and
Matt Bishop's "Computer Security: Art and Science".
A good modern formal text at the introductory level is
Goodrich - Tamassia "Introduction to Computer Security"
https://www.amazon.com/dp/0321512944/?tag=pfamazon01-20that does not assume extensive knowledge in CS or math and
Matt Bishop's "Introduction to Computer Security"
https://www.amazon.com/dp/0321247442/?tag=pfamazon01-20. Of course, as
jedishrfu points out,
google is your friend and there is plenty of useful information you can find out, according to your goals. I'll stress out that good knowledge about computer networks, some decent coding skills and good skills of using probing / testing tools and suites both command - line oriented as well as GUI oriented, are required in order to pursue the subject beyond the introductory level.