Linux Inside – How the Linux Kernel Works
Besides some scattered text files and the Linux man pages, there is this: <a href="https://www.kernel.org/doc/html/latest/" rel="nofollow">https://www.kernel.org/doc/html/latest/</a><p>It's fairly new - started about a year ago, but there's quite a lot of stuff in there already (bootstrapped from the old XML stuff, I believe).