Security Engineering: A Guide to Building Dependable Distributed Systems by Ross Anderson is one of, if not the best information security book ever written.
With a list price of $80, it's worth every penny.
With that, thanks to Robert Slade for pointing out today in Risks Digest 27.16 that Ross Anderson has made all chapters from the second edition now available free online. You can get it here.
The hardcopy version on Amazon is available here.
Bruce Schneier said of Security Engineering “it's beautiful. This is the best book on the topic there is”. Enough said.