Those who do not understand UNIX are condemned to reinvent it, poorly.
Edited by Peter Denning, Computers Under Attack: Intruders, Worms, and Viruses, ACM Press, 1990, 0-201-53067-8.
This compendium contains a couple of articles on shell script viruses.
Ken Burtch, Linux Shell Scripting with Bash, 1st edition, Sams Publishing (Pearson), 2004, 0672326426.
Covers much of the same material as the ABS Guide, though in a different style.
Dale Dougherty and Arnold Robbins, Sed and Awk, 2nd edition, O'Reilly and Associates, 1997, 1-156592-225-5.
Unfolding the full power of shell scripting requires at least a passing familiarity with sed and awk. This is the standard tutorial. It includes an excellent introduction to Regular Expressions. Recommended.
Jeffrey Friedl, Mastering Regular Expressions, O'Reilly and Associates, 2002, 0-596-00289-0.
Still the best all-around reference on Regular Expressions.
Aeleen Frisch, Essential System Administration, 3rd edition, O'Reilly and Associates, 2002, 0-596-00343-9.
Stephen Kochan and Patrick Wood, Unix Shell Programming, Hayden, 1990, 067248448X.
Still considered a standard reference, though somewhat dated, and a bit "wooden" stylistically speaking.  In fact, this book was the ABS Guide author's first exposure to UNIX shell scripting, lo these many years ago.
Neil Matthew and Richard Stones, Beginning Linux Programming, Wrox Press, 1996, 1874416680.
Surprisingly good in-depth coverage of various programming languages available for Linux, including a fairly strong chapter on shell scripting.
Herbert Mayer, Advanced C Programming on the IBM PC, Windcrest Books, 1989, 0830693637.
Excellent coverage of algorithms and general programming practices. Highly recommended, but unfortunately out of print.
David Medinets, Unix Shell Programming Tools, McGraw-Hill, 1999, 0070397333.
Pretty good treatment of shell scripting, with examples, and a short intro to Tcl and Perl.
Cameron Newham and Bill Rosenblatt, Learning the Bash Shell, 2nd edition, O'Reilly and Associates, 1998, 1-56592-347-2.
This is a valiant effort at a decent shell primer, but sadly deficient in its coverage of writing scripts and lacking sufficient examples.
Anatole Olczak, Bourne Shell Quick Reference Guide, ASP, Inc., 1991, 093573922X.
A very handy pocket reference, despite lacking coverage of Bash-specific features.
Jerry Peek, Tim O'Reilly, and Mike Loukides, Unix Power Tools, 3rd edition, O'Reilly and Associates, Random House, 2002, 0-596-00330-7.
Contains a couple of sections of very informative in-depth articles on shell programming, but falls short of being a self-teaching manual. It reproduces much of the Regular Expressions tutorial from the Dougherty and Robbins book, above. The comprehensive coverage of UNIX commands makes this book worthy of a place on your bookshelf.
Clifford Pickover, Computers, Pattern, Chaos, and Beauty, St. Martin's Press, 1990, 0-312-04123-3.
A treasure trove of ideas and recipes for computer-based exploration of mathematical oddities.
George Polya, How To Solve It, Princeton University Press, 1973, 0-691-02356-5.
The classic tutorial on problem-solving methods (algorithms), with special emphasis on how to teach them.
Chet Ramey and Brian Fox, The GNU Bash Reference Manual, Network Theory Ltd, 2003, 0-9541617-7-7.
This manual is the definitive reference for GNU Bash. The authors of this manual, Chet Ramey and Brian Fox, are the original developers of GNU Bash. For each copy sold, the publisher donates $1 to the Free Software Foundation.
Arnold Robbins, Bash Reference Card, SSC, 1998, 1-58731-010-5.
Excellent Bash pocket reference (don't leave home without it, especially if you're a sysadmin). A bargain at $4.95, but unfortunately no longer available for free download.
Arnold Robbins, Effective Awk Programming, Free Software Foundation / O'Reilly and Associates, 2000, 1-882114-26-4.
The absolute best awk tutorial and reference. The free electronic version of this book is part of the awk documentation, and printed copies of the latest version are available from O'Reilly and Associates.
This book served as an inspiration for the author of the ABS Guide.
Bill Rosenblatt, Learning the Korn Shell, O'Reilly and Associates, 1993, 1-56592-054-6.
Paul Sheer, LINUX: Rute User's Tutorial and Exposition, 1st edition, , 2002, 0-13-033351-4.
Very detailed and readable introduction to Linux system administration.
The book is available in print, or on-line.
Ellen Siever and the staff of O'Reilly and Associates, Linux in a Nutshell, 2nd edition, O'Reilly and Associates, 1999, 1-56592-585-8.
The all-around best Linux command reference. It even has a Bash section.
Dave Taylor, Wicked Cool Shell Scripts: 101 Scripts for Linux, Mac OS X, and Unix Systems, 1st edition, No Starch Press, 2004, 1-59327-012-7.
Pretty much what the title promises . . .
The UNIX CD Bookshelf, 3rd edition, O'Reilly and Associates, 2003, 0-596-00392-7.
An array of seven UNIX books on CD ROM, including UNIX Power Tools, Sed and Awk, and Learning the Korn Shell. A complete set of all the UNIX references and tutorials you would ever need at about $130. Buy this one, even if it means going into debt and not paying the rent.
Update: Seems to have somehow fallen out of print. Ah, well. You can still buy the dead-tree editions of these books.
Fioretti, Marco, "Scripting for X Productivity," Linux Journal, Issue 113, September, 2003, pp. 86-9.
Ben Okopnik's well-written introductory Bash scripting articles in issues 53, 54, 55, 57, and 59 of the Linux Gazette, and his explanation of "The Deep, Dark Secrets of Bash" in issue 56.
Chet Ramey's Bash - The GNU Shell, a two-part series published in issues 3 and 4 of the Linux Journal, July-August 1994.
William Park has been working on a project to incorporate certain Awk and Python features into Bash. Among these is a gdbm interface. He has released bashdiff on Freshmeat.net. He has an article in the November, 2004 issue of the Linux Gazette on adding string functions to Bash, with a followup article in the December issue, and yet another in the January, 2005 issue.
Tim Waugh's xmlto is an elaborate Bash script for converting Docbook XML documents to other formats.
Of historical interest are Colin Needham's original International Movie Database (IMDB) reader polling scripts, which nicely illustrate the use of awk for string parsing. Unfortunately, the URL link is broken.
Penguin Pete has quite a number of shell scripting tips and hints on his superb site. Highly recommended.
The excellent Bash Reference Manual, by Chet Ramey and Brian Fox, distributed as part of the bash-2-doc package (available as an rpm). See especially the instructive example scripts in this package.
The manpages for bash and bash2, date, expect, expr, find, grep, gzip, ln, patch, tar, tr, bc, xargs. The texinfo documentation on bash, dd, m4, gawk, and sed.
It was hard to resist the obvious pun. No slight intended, since the book is a pretty decent introduction to the basic concepts of shell scripting.