Technical book publisher Apress has made available the complete text of several of its older titles as free e-books. I'm surprised to see Practical Common Lisp [PDF] by Peter Seibel, in this list, which I thought was one of their bigger sellers. But for good books, nothing beats a bound paper book, so it's unlikely to detract from sales. (And for bad books, nothing beats a free e-book.)

I consider Practical Common Lisp successful because it seemed like most people I know who are interested in the subject matter considered it a worthy successor to Paul Graham's On Lisp as the canonical first Lisp book. Well, now you can see for yourself, because Paul has made On Lisp available as a free download.