When I started to learn GLSL I had a hard time finding the information that I wanted to learn online. There is/was a lot of basic examples, but very little material that was in depth; for example passing and processing an array of light data for an object. I found that the OpenGL SuperBible had a great introduction to the language, but it too was not enough. There were only a handful of books on the market that did any real explaining of the language. However, I was lucky to have found a great book on my first purchase the OpenGL 4 Shading Language Cookbook by David Wolff.
The book takes a literal standing by the title with each section containing a GLSL recipe. The book has over 70 of these recipes with explanations of how each program operates. The author does a very good job getting right to the point within each section starting with a handful of paragraphs and then the GLSL code. By the end of chapter two I found myself already seeing wonderful results within my work by following along with his examples.
In addition to the material within the book the author maintains a repository of the example source code for download by his readers. The repository (at the time of my visit) contains the example code of both the first edition and second editions of his book.
I do recommend this book for both the beginner and the advance user. I also recommend keeping this book close to your work area because it is perfect for a quick reference.