Here are some resources that OpenACS users have found useful.
Philip and Alex's Guide to Web Publishing - A very readable guide to database-backed community websites.
UNIX Power Tools - An excellent introduction to the command line tools and basic programs of UNIX
UNIX System Administration Handbook (formerly the "red book" - now the "purple" book)
UNIX System Administrator's Bible - (LePage and Iarerra 1998; IDG)
LPI certification exam preps - A series of articles from IBM developerworks on basic and intermediate Linux skills (requires registration)