Common Lisp interpreter

I'm reading Land of Lisp and the guy uses CLISP (and CLISP specific extensions), so I'm using it for now. But I've heard good things about SBCL and others. Which one would you recommend? Should I stick with CLISP?

Trivia: clisp turned GPL because of readline.
Wtf is up with that readline thing? Why they chose GPL, and why does everything (like PHP) apparently have an option for it?