"Linux in a Nutshell" by Ellen Siever, Jessica P. Hackman, Stephen Spainhour, Stephen Figgins, O'Reilly UK, ISBN 0596000251
"Running Linux" by Matt Welsh, Matthias Kalle Dalheimer, Lar Kaufman, O'Reilly UK, ISBN 156592469X
"Linux Unleashed" by Tim Parker, Bill Ball, David Pitts, Sams, ISBN 0672316889
"When You Can't Find Your System Administrator" by Linda Mui, O'Reilly UK, ISBN 1565921046
When you actually buy a distribution, it will contain a very decent user manual.
"Learning the Vi Editor" by Linda Lamb and Arnold Robbins, O'Reilly UK, ISBN 1565924266
"GNU Emacs Manual" by Richard M.Stallman, iUniverse.Com Inc., ISBN 0595100333
"Learning GNU Emacs" by Debra Cameron, Bill Rosenblatt and Eric Raymond, O'Reilly UK, ISBN 1565921526
"Perl Cookbook" by Tom Christiansen and Nathan Torkington, O'Reilly UK, ISBN 1565922433
"Unix Shell Programming" by Stephen G.Kochan and Patrick H.Wood, Sams Publishing, ISBN 067248448X
"Learning the Bash Shell" by Cameron Newham and Bill Rosenblatt, O'Reilly UK, ISBN 1565923472
"The Complete Linux Shell Programming Training Course" by Ellie Quigley and Scott Hawkins, Prentice Hall PTR, ISBN 0130406767
"Linux and Unix Shell Programming" by David Tansley, Addison Wesley Publishing Company, ISBN 0201674726
"Unix C Shell Field Guide" by Gail and Paul Anderson, Prentice Hall, ISBN 013937468X
"Gnome User's Guide" by the Gnome Community, iUniverse.Com Inc., ISBN 0595132251
"KDE Bible" by Dave Nash, Hungry Minds Inc., ISBN 0764546929
"The Concise Guide to XFree86 for Linux" by Aron HSiao, Que, ISBN 0789721821
"The New XFree86" by Bill Ball, Prima Publishing, ISBN 0761531521
"Beginning GTK+ and Gnome" by Peter Wright, Wrox Press, ISBN 1861003811
"KDE 2.0 Development" by David Sweet and Matthias Ettrich, Sams Publishing, ISBN 0672318911
"GTK+/Gnome Application Development" by Havoc Pennington, New Riders Publishing, ISBN 0735700788
"TCP/IP Illustrated, Volume I: The Protocols" by W. Richard Stevens, Addison-Wesley Professional Computing Series, ISBN 0-201-63346-9
"DNS and BIND" by Paul Albitz, Cricket Liu, Mike Loukides and Deborah Russell, O'Reilly & Associates, ISBN 0596001584
"The Concise Guide to DNS and BIND" by Nicolai Langfeldt, Que, ISBN 0789722739
"Implementing LDAP" by Mark Wilcox, Wrox Press, ISBN 1861002211
"Understanding and deploying LDAP directory services" by Tim Howes and co., Sams, ISBN 0672323168
"Sendmail" by Brian Costales and Eric Allman, O'Reilly UK, ISBN 1565922220
"Removing the Spam : Email Processing and Filtering" by Geoff Mulligan, Addison Wesley Publishing Company, ISBN 0201379570
"Managing IMAP" by Dianna & Kevin Mullet, O'Reilly UK, ISBN 059600012X