You've gotten some other good responses, but I thought I'd mention the
LoC and OCLC sites on MARC if you haven't seen them yet.
First, the LoC site at http://www.loc.gov/marc/. This is what I use
as a guide and a reference.
Some folks prefer the OCLC docs http://www.oclc.org/bibformats/en/,
particularly if they're an OCLC member.
Of course, these apply to MARC-21 and not UniMarc. Not sure what good
resources are out there for UniMARC.