Ross Anderson's Security Engineering is an excellent introductory book. Highly readable, and broad but not very deep. The first edition is free online (and is

Ross Anderson's Security Engineering is an excellent introductory book. Highly readable, and broad but not very deep. The first edition is free online (and is still a perfectly resource; the second edition has a few added chapters): http://www.cl.cam.ac.uk/~rja14/book.html http://www.cl.cam.ac.uk/~rja14/book.html