This is easily the best book I've seen for learning C. King does a great job of explaining why things work the way they do. If you don't already have this book, I highly recommend getting a copy.
I have not yet found a really good introductory Perl book. This one is okay. I'm hoping the 3rd edition is better, but I haven't seen it. (Please let me know if you have a Perl book that you like.)
We do not cover web programming with Perl in this course, but Castro does a good job of explaining the basics of Perl.
Generally considered "the" C reference book. It can be a bit hard to read and digest. There is an online version of the book. (I'm not sure if it is an officially sanctioned version.)
This book focuses on how to use Unix rather than on how to program it. It goes into much more detail than Glass and Ables and at roughly 900 pages you would hope that it would. Generally, it seems quite readable and sensible.
We have used this book as a text in csc209. It is fairly good at teaching shell programming and Unix utilities, but has relatively little on the systems programming topics.
I quite like the Visual QuickStart Guides from Peachpit Press. They feature step-by-step instructions for how to do many things. This book is aimed at the beginning Unix user. The authors do a good job of explaining the reasons behind the steps that they give you.
Stevens' books are fantastic references for systems programming in Unix. They geared more at someone already familiar with Unix and familiar with programming, but are very readable. The course text covers similar material at more of a tutorial level.
If you are ever going to buy a book on network programming, this is the one to buy.
If you find a web page that you think would be useful to other students in the class, send me mail and I'll add a link here.