More important than the acronym is the effort itself. XINU is a really interesting project and probably the second OS I was introduced to outside of the POSIXy neighbourhood. It differs from a lot of other projects in that its goals (to my understanding) were very different to the usual approach toward providing POSIX compliance or a platform intended to support layering POSIX on top of a base architecture.
I picked up an old copy of the textbook and never read it. Maybe this is the kick I needed to pick up the newer edition and actually check it out in earnest!
When I first learned C I also read Lions' Notes on v6 Unix, and kind of saw what was going on, but the XINU book really drove home how kernels worked. The XINU book on internetworking was similarly great.
I picked up an old copy of the textbook and never read it. Maybe this is the kick I needed to pick up the newer edition and actually check it out in earnest!