>>1
That depends on what you know. Since you want to learn CL specifically, I assume you already have some programming experience, in which case Practical Common Lisp is the book for you. If you're new to programming, A Gentle Introduction To Symbolic Computation might be a better choice. Google will find you both online.
Some suggest that Scheme is a better intro to Lisp, but I disagree. Lisp is as imperative as it is functional. It's got five magnificent looping constructs for a reason (and Iterate and Series lurking on the web). IMO, if you're writing your iteration recursively in Common Lisp, you're fucking around because you like it, not because it's encouraged by or helpful in the language. Starting with Scheme is going to get its weirdness mixed in with your notion of how to Lisp productively.
IMO, coming to grips with macros (when to use them and when
not to use them) and CL's other powerful features (such as the condition system and CLOS) is at least as important as programming in a functional style. One of the great things about Lisp is that you don't need to write a function to return a value. A conditional, a loop, or a progn will do as well. The important thing is that you learn to write programs declaratively and develop helpful abstractions to do it.