After reading some articles by Paul Graham I decided to do something about acquiring some proficiency in Lisp. I'll probably never use it at work but Lisp does seem such a fundamental part of the history of computing that I feel I'm lacking as a software engineer without it.
Searching via Google I found some recommendations common to several sources. Successful Lisp by David Lamkins is an online book which I started reading last night. So far I've found it an excellent introductory text. I've also ordered the following books from Amazon: ANSI Common LISP by Paul Graham, and Common Lispcraft by Robert Wilensky.
[Sucessful Lisp is only available online but I used the excellent wget to mirror it onto my laptop. The Windows version worked fine once I had remembered to configure the proxy server setting.]