I was wondering how this compares with xv6, which is a Unix v6 like 32 bit system written for teaching so peeked at the book and the code. I've only scratched the surface but the developer of the project says that he's deliberately written the code in a very simple style to make its function clear and obvious. I'd agree. It does seem pretty easy to understand. It's also very much simpler in structure and the facilities that xv6 in terms of the facilities that it offers. Fun project and it might be a good place those wanting to understand operating systems to start. Here's the repo for it.
539 appears to only be a kernel, there isn’t any userspace. Xv6 includes a kernel as well as a userspace containing well known utilities like sh, ls, cat, and others.
https://github.com/MaaSTaaR/539kernel