We used the book in my undergrad networking course, and I liked it quite a bit. It's "systems" in the catchall "lower-ish-level computing" sense of "systems-level programming," rather than the "systems theory" sense.

The pedagogical structure of the book makes it a solid follow-up to a standard undergrad OS class, IIRC. It's been ten years since I've read it, though, so YMMV.