Mastering Unix Shell Scripting: Bash, Bourne, and Korn Shell Scripting for Programmers, System Administrators, and UNIX Gurus 2nd (second) Edition by Michael, Randal K. published by Wiley
One of the better programming 'How To' texts I've ever used. At the time my focus was on Korn shell, this book does a good job of translating the various shells with similar examples and I like examples. There is also an online stash of example scripts you can download.
I still use it as a reference and reminder to what i wrote back when, mmm what did I mean by that line?
You will need to start somewhere. This “Mastering UNIX Shell Scripting by Randy Michael” gives a fair introduction to the subject.
Some scripters will find this too comprehensive and need to supplement it with other reading. Others will find this too fundamental but occasionally there is a good idea that was not thought of.
Some people just need a guide to help them get off the ground fast. No matter what category you are in this is an excellent reference for your library.