Practical Common Lisp lives up to its name as the most practical of introduction texts, covering all the core topics and including many small example projects.
Touretzky's Gentle Introduction is popular, but targeted more at people entirely new to programming. The nice thing about this book is it is more like a college textbook in that it has many homework questions with answers in the back, so its great for structured practice.
Paul Graham's ANSI Common Lisp is still my favorite introductory text assuming the reader has knowledge of an existing functional language (using Javascript functionally for example). The books is a firehose of information, is short yet somehow manages to cover even more material than the other books, includes a full reference to CL, has a section on performance tuning I haven't seen in other intro texts, and still fits in meaty examples like a raytracer and a prolog-lite DSL. It's a real shame this book does not get a reprint, you'll need to buy a used version to get it at a decent price, however I'm finding myself using it as a reference so much I'm considering buying a new copy.
What I've found with learning CL as opposed to other langauges is that I get a lot of value out of reading multiple beginner texts. Rather than having a small number of core concepts like an OO language, CL just has a massive number of functions. This makes it nicer to work with IMO but just takes more repetition to learn.
An alternative approach I would love to see somebody try is to learn straight from Norvig's Paradigms of AI Programming. The book has a brief introduction to the langauge at the start, then goes into writing actual very interesting code, then ends with a section covering some of the more detailed topics. If you're the kind of person who learns most by example of real-world code, it could work.
