I bought this "book" because I am writing a program that deals extensively with strings and text (natural language parsing). I was having some difficulty mastering sed, and understanding how it works. For example, "s/[^A-Za-z]//" was not working (to remove non alpha chars). After five pages in this little gem, I knew what I needed. A trailing "g".
The book is rich with examples, explanations, and really just cuts straight to the chase.
This is not a book for the casual user. It is, after all, a reference, not a tutorial. Unix users (and administrators) who already have a basic grasp of regular expressions and just what sed and awk are (and grep/egrep too) will find this most rewarding.
While I have a hard time paying this much for what amounts to less than toaster oven instructions, it is absolutely invaluable to me, and it doesnt take up space on my desk or bookshelf.