xv6 is pretty much the best osdev beginner's ressource out there. Once you've mastered it, "Operating Systems: Design and Implementation" is still a worthwhile read even though it's getting a bit old.

After that, it depends on what you find interesting. Personally, I think Unix is a fossilized relic whose 50 year old design has been pushed far beyond its use-by date. I'm a fan of Fuchsia OS and especially its Zircon kernel, its design documentation is well worth a read just to shake off the idea that POSIX makes for a great syscall layer in the 21st century.

In the end, it doesn't matter too much, you'll always learn something no matter what the operating system you look into. Try things out and discover what you like instead of relying on what others find interesting. I just wish that people stop thinking that the traditional, monolithic Unix system design is some sort of holy scripture that shall not be questioned.