Ross Anderson, Professor of Security Engineering at the Computer Laboratory of The University of Cambridge has just published the second edition of his book Security Engineering, the preface and six chapters of which is available to download.
To quote Bruce Schneier “This is the best book on the topic there is, and I recommend it to everyone working in this field — and not just because I wrote the foreword.”
Professor Anderson has made the first edition of his fantastic book available to download for free as a 17MB pdf.