One thing the Linux kernel has going for it is that there are a lot of books that describe how the various parts work and how to use the various internal interf

One thing the Linux kernel has going for it is that there are a lot of books that describe how the various parts work and how to use the various internal interfaces. I can't think of any other open source project that has multiple books on how to contribute. (Sadly, most of those good kernel books were written in the 90's and early 2000's. I don't know if there are any recent kernel hacking books.)