Not only is the book free to read (although I would suggest to pay for it if you like it!) The code to parse binary files actually "inspired" my design of an actual production application which was very flexible, succinct and, most importantly, so fast I had to spend a lot of effort convincing people the numbers are actually true.
It taught me some important lessons about how you can achieve performance with Lisp languages and the real reasons for the power of macros. Not too shabby for the first book on Lisp I red!