From time to time Grandpa Rob @rslade or I or some other forum member will recommend reading Ross Anderson's book Security Engineering. While it is well worth buying the massive dead-tree tome (I did twice), you can download for free the entire book 2001 1st edition, 2008 2nd edition, and the in-progress (2019-2020) draft 3rd edition, chapter-by-chapter at the linked site.
This note is to encourage all to download the 3rd edition now, and watch that page for final chapters. It appears Anderson is getting close to completion; Chapter 21 showed up on March 16, 2020.. Significantly, on that page he told us,
"Once the manuscript's finished and goes to press, all except seven sample chapters will disappear for a commercial period of 42 months. I'm afraid the publishers insist on that. But therearefter the whole book will be free online forever."
I am sure both Anderson and his publisher would love for us to buy the 3rd edition in hardcover, but if you want your own stored free version, better get it while available.
Oh, yeah, and after downloading, be sure to actually read it!