My standard recommendation: begin with the SICP videos, which you can find at http://groups.csail.mit.edu/mac/classes/6.001/abelson-sussma...
I can't think of a more effective way to invest 20 hours.
I can't think of a more effective way to invest 20 hours.
But although SICP teaches you a great deal about Lisp techniques, it doesn't teach Common Lisp. Abelson and Sussman use Scheme. If you are aware of the differences, it should be no problem, though.
This has been written 10k times here: The video lectures are great, the book is great (It's also free on the internet)! If you don't depend on Common Lisp, this is probably the best Lisp (and programming in genereal) introduction out there. As a follow-up, Paul Graham's "On Lisp" is a great choice.