UNIX-based administrators may want to consult UNIX for OpenVMS Users, Second Edition, by Philip Bourne, Richard Holstein, and Joseph McMullen from Digital Press. The title implies that you must know VMS to learn UNIX but I think the book can be used in the opposite sense as well.
Windows NT-based administrators may want to look at Windows NT for OpenVMS Professionals by David Solomon published by Digital Press. Like the UNIX book above it can be used to relate Windows NT commands to OpenVMS.
No matter what your background, The OpenVMS User's Guide is probably a good investment too. It describes the basics of using OpenVMS, its primary commands and tools.
The comp.os.vms discussion group is mirrored on INFO-VAX, a list server. To subscribe to INFO-VAX, send e-mail to <INFO-VAX-REQUEST@mvb.saic.com> with the message SUBSCRIBE INFO-VAX. This is a very active list with around 100 messages posted per day. Like any list, not all messages pertain to OpenVMS technical discussions though.
All online documentation is rooted at www.openvms.compaq.com:8000. This page contains links for OpenVMS manuals for several versions. Some manuals are only available in HTML others in PDF and many in both formats. Of course the documents are available from Compaq/HP in hard copy and CD formats as well.
The OpenVMS Version 7.3 New Features and Documentation Overview includes a section describing all the manuals available from Compaq, including order numbers.