Ross Anderson has convinced his publisher that the world won't come to an end if he has a copy of his book, Security Engineering, available for free on the web:
http://www.cl.cam.ac.uk/~rja14/book.html
Its an all-around excellent book. It's not just about software security (though software is a recurring topic), it's about how to build secure systems. It is just chock full of examples ranging from ATM security to aircraft identify-friend-or-foe systems, to nuclear weapon command and control. Now that you can download it for free, there are no more excuses. Read it!
