This is THE bible for awk users. If you refuse to use perl, which incorporated most of awk, or have to maintain old awk scripts then this is the text. I like the O'Reilly text for learning the basics, but after that you need this guide. For some tasks awk is the premier solution, for others, use awk as part of the solution along with perl and a shell script. Still a cool language. And this book is very helpful for both beginning intermediate programmers of awk and advanced users.

The AWK Programming Language was written very well. The first couple of chapters describe the language and the rest are full of real-world examples. The exercises at the end of each section are very helpful and expand on the examples given. Any one who works with flat data files should be familiar with awk, and this is the book to get the job done.

While the Nutshell book is more of a reference guide, Aho's shows how to really use the language with practical examples.
From basic examples to flatfile reports and using Awk to try out language issues in compiler design.
For those who know Awk there are some great ideas in here. For those who are just starting out it's an excellent way to ease yourself into writing Awk scripts.
A script writers must have.