Unix System V Release 4: An Introduction. Rosen, Kenneth H. (ed.), Richard R. Rosinsi, and James M. Farber, 1996.
Learning the UNIX Operating System (A Nutshell Handbook), Todino, Grace, and John Strang, 1994.
O'Reilly and Associates series of Nutshell handbooks on UNIX topics.
UNIX for People: A Modular Guide to the UNIX Operating System: Visual Editing, Document Preparation, and Other Resources. Birns, Peter, Patrick Brown, and John Muster, 1984.
UNIX Primer Plus, Waite, Mitchell, Donald Martin, and Stephen Prata, 1990.
New to UNIX but have technical background
The UNIX Programming Environment, Kernighan, Brian W., and Robert Pike, 1984.
Introducing UNIX System V, Morgan, Rachel, and Henry McGilton, 1987.
Advanced
UNIX Power Tools, Peek, Jerry D., 1993.
Advanced UNIX Programming, Rochkind, Marc J., 1985.
Advanced Programmer's Guide to UNIX System V, Thomas, Rebecca, Lawrence Rodgers, and Jean Yates, 1986.
The Design of the UNIX Operating System, Bach, Maurice J., 1986.
Shell programming
The UNIX C Shell Field Guide, Anderson, Gail, and Paul Anderson, 1986.
UNIX Shell Programming, Kochan, Stephen G., and Patrick Wood, 1990.
UNIX Shell Programming, Arthur, Lowell Jay, 1990.
UNIX system manuals (known as the "man pages")
UNIX Programmer's Manual, Vol. 1, 1986.
UNIX Programmer's Manual, Vol. 2, 1986.
UNIX Programmer's Manual, Vol. 3, 1986.
UNIX Programmer's Manual, Vol. 4, 1986.
UNIX Programmer's Manual, Vol. 5, 1986.
UNIX System Administrator's Reference Manual, 1988.
UNIX System User's Manual, 1987.
4.3BSD UNIX Manuals.
Unix- Frequently Asked Questions