If you're the kind of person who likes to know how things work down to the bits, Lisp in Small Pieces by Christian Queinnec (Cambridge University Press, 1996) provides a nice blend of programming language theory and practical Lisp implementation techniques. While it's primarily focused on Scheme rather than Common Lisp, the same principles apply.
For folks who want a little more theoretical look at things—or who just want to know what it's like to be a freshman comp sci student at M.I.T.—Structure and Interpretation of Computer Programs, Second Edition, by Harold Abelson, Gerald Jay Sussman, and Julie Sussman (M.I.T. Press, 1996) is a classic computer science text that uses Scheme to teach important programming concepts. Any programmer can learn a lot from this book—just remember that there are important differences between Scheme and Common Lisp.
Once you've wrapped your mind around Lisp, you may want to place it in a bit of context. Since no one can claim to really understand object orientation who doesn't know something about Smalltalk, you might want to start with Smalltalk-80: The Language by Adele Goldberg and David Robson (Addison Wesley, 1989), the standard introduction to the core of Smalltalk. After that, Smalltalk Best Practice Patterns by Kent Beck (Prentice Hall, 1997) is full of good advice aimed at Smalltalkers, much of which is applicable to any object-oriented language.
And at the other end of the spectrum, Object-Oriented Software Construction by Bertrand Meyer (Prentice Hall, 1997) is an excellent exposition of the static language mind-set from the inventor of Eiffel, an oft-overlooked descendant of Simula and Algol. It contains much food for thought, even for programmers working with dynamic languages such as Common Lisp. In particular, Meyer's ideas about Design By Contract can shed a lot of light on how one ought to use Common Lisp's condition system.
Though not about computers per se, The Wisdom of Crowds: Why the Many Are Smarter Than the Few and How Collective Wisdom Shapes Business, Economies, Societies, and Nations by James Surowiecki (Doubleday, 2004) contains an excellent answer to the question, "If Lisp's so great how come everybody isn't using it?" See the section on "Plank-Road Fever" starting on page 53.
And finally, for some fun, and to learn about the influence Lisp and Lispers have had on hacker culture, dip into (or read from cover to cover) The New Hacker's Dictionary, Third Edition, compiled by Eric S. Raymond (MIT Press, 1996) and based on the original The Hacker's Dictionary edited by Guy Steele (Harper & Row, 1983).
But don't let all these suggestions interfere with your programming—the only way to really learn a language is to use it. If you've made it this far, you're certainly ready to do that. Happy hacking!