Well, if you want the word of someone with VERY limited experience, I would recommend Norvig's "Paradigms of Artificial Intelligence Programming." He has a couple of excellent chapters on efficiency issues. Actually most of the book could be considered something of a style guide, or Guide to Writing Good Lisp, because of the way that Dr. Norvig takes complex problems (from the AI community) and describes the process of writing solutions in Lisp. I had warm fuzzy feelings about Lisp before reading this book, but came away feeling nearly evangelical about Lisp's place in the world. Luckily my fine sense of pragmatism and irony keeps me from going medieval on my Visual C++ and Visual Basic (shudder) fellow workers.