This does seem to be a seriously good book for learning Lisp. (From somebody who is trying to, and is finding it very helpful!)
Visit the URL mentioned above for the latest news. As of today, March 29, 2005, it says Seibel has reached an agreement with Apress that he will be able to keep the full text online. I think this may turn out best for sales because more people will read the book and be able to recommend the book to beginners.
It's the book that finally taught me Lisp --StevenNewton