Killer app? Franz Inc. has a long list of success stories, but this list only includes users of AllegroCL... There are probably others. My favourite is the story about Naughty Dog, since I was a big fan of the Crash Bandicoot games.
For learning Common Lisp, I'd recommend Practical Common Lisp. It has a hands-on approach that at least for me made it easier than other books I've looked at.
