This is usually up to the publisher, in cases where the author has signed a traditional publishing deal. It looks like Prentice Hall has taken On Lisp out of p

This is usually up to the publisher, in cases where the author has signed a traditional publishing deal. It looks like Prentice Hall has taken On Lisp out of print. After publishers do that, they often let the author post a version online, since they are no longer selling it anyway. ANSI Common Lisp, on the other hand, is still in print.