Ross Anderson put the entire second edition of Security Engineering on his website for free download. It's been very highly rated and recommended. Each chapter comes in its own PDF so be prepared to use a downloader to grab it all.
An interactive visual search engine. An interactive mind map that links to other mind maps as well as sites. Designed to be explored but can also be searched.
Public instance here: https://learn-anything.xyz/
3712 links, including 188 private