I actually found Practical Common Lisp
to be very useful. Prior to that, I'd done various bits of scripting and programming in unix shell, SQL, Perl and PHP, so I wasn't a total beginner to programming. However, it's the best introduction to Lisp itself I've found.
After that, it'll be worth looking up Paul Graham's On Lisp
,and the Hyperspec, which is available from Lispworks