Lisp suffers a bit from being too early to the party. When someone wants to learn a new programming language (one that's not literally a new one), there is usually a single obvious canonical implementation (most likely already available on all Unix-like machines) hosted on [language].org, with a set of documents that describe it.

Lisp is (and many other pioneering languages are) no such beast. Lisp.org is McCarty's page (peace be upon Him) and my Linux laptop has about half a dozen dialects of Lisp: Emacs Lisp one or two CL's, Guile, Clojure and Scheme. I love it, but it wasn't easy to start.