If you have a serious interest in system and network security, one of the best reference books available is Security Engineering, by Ross Anderson. The first edition of this book was published in 2001, and quickly became a standard text. Under an agreement with the publishers, the complete text of that book was made freely available online, four years after its publication. The second edition, which I think is even better, was published in 2008; as with the first edition, the second edition text is now available online.
Dr. Anderson is Professor of Security Engineering at the Computer Laboratory, University of Cambridge, and has for many years been regarded as one of the world’s top security experts. He received his PhD from Cambridge, and is a Fellow of the Royal Society, the Institute of Physics, and the Royal Academy of Engineering. His home page (linked above) has an overview of his very extensive research work.
My (paper) copy of the first edition of Security Engineering is one of the handful of reference books I use all the time. I don’t have a paper copy of the second edition yet, but I’ll certainly be ordering one. If you have a serious interest in the field, I recommend it without reservation.