A Practical Guide to The UNIX System, 3rd Edition. Addison Wesley, 1994.
Sobell, Mark G.
This is the book that a lot of people have used to learn Unix, which makes sense, because it is written in a very clear style, which is good when you're dealing with what could be a very complicated subject. As far as Unix distributions, this book covers SunOS 4.1.3 and BSD 4.3. Sobell's book is divided into two parts. Part I covers the system in general, focusing on programming, the Internet, & other applications, while part II examines more than 75 Unix utility programs. Essential.

UNIX: Visual QuickStart Guide. Peachpit Press, 1998.
Ray, Deborah S. & Eric J. Ray.
Another in the line of Visual QuickStart Guides, this is one of the best. It's long & it is very, very useful. If you're just learning Unix, this is a fabulous book. If you'd like a more advanced book, though, look at Sobel's A Practical Guide to The Unix System.