OS/390 and z/OS Freeware
ABCs of Systems Programming
God Bless America
IBM has released two Redbook series entitled the "ABCs of Systems Programming". One is a five volume set for OS/390 Systems Programming and the other is a Eleven volume set. The stated goal of the volumes are: "to introduce the structure of OS/390, z/OS, and S/390 operating environment. The set will help you install, tailor, and configure an OS/390 operating system, and is intended for system programmers who are new to an OS/390 or z/OS environment."
These volumes, and more, can be found at http://www.redbooks.ibm.com or read the individual books at:
ABCs of z/OS Systems Programming
Volume 2 z/OS Implementation and Maintenance
Volume 7 InfoPrint Server, LE, and SMP/E
Volume 8 z/OS Problem Diagnosis
Volume 9 z/OS, UNIX System Services
Volume 10 Intro to Z, LPAR, and HCD
Volume 11 Capacity planning, performance management, RMF, and SMF
Volume 12 WLM
Volume 13 JES3
ABCs of OS/390 Systems Programming (historical)
IBM has made available the IBM Systems Journals electronically - check them out.
For the history buff check out this scanned copy of an OS/360 system chart.