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.
[…]I would also like to thank MtoM and Derrick in helping me shape my career and would just like to wish him and the company all the best.[…]