Bob DuCharme has made his excellent Operating Systems Handbook available online in PDF format — containing great introductions to more arcane OSs such as VM or MVS (I have used the former in the past, but never had a chance to actually do anything with z/OS neé MVS). Highly recommended.
This is a single archived entry from Stefan Tilkov’s blog. For more up-to-date content, check out my author page at INNOQ, which has more information about me and also contains a list of published talks, podcasts, and articles. Or you can check out the full archive.