I brought Lisp System Implementation a few weeks ago. While its pretty good, I did have one feedback: it would be great to have a book that shows making a Lisp step-by-step, rather than documenting the final version. I know a few people who are interested in learning how to write compilers, and would prefer something like that (LSI would be too hard for them, since it would require reading a large portion of the book to even gain some basic context of why we are doing what we are doing).