Basics

• Books:
  • Practical Common Lisp
    Good all-around introduction.
  • Paradigms of Artificial Intelligence Programming
    Dead-tree only. Covers quite a few interesting topics, including optimization. However, one caveat is that he is occasionally willing to run out of stack or exceed maximum number of arguments to a function.
  • Successful Lisp
  • Common Lisp: A Gentle Guide to Symbolic Computation
    This one is very good if you are new to programming in Lisp-like languages (probably includes python and club)
  • ANSI Common Lisp
    There is an online version. Has a very good reference in the back. Unfortunately, biased against OOP and LOOP, in favor of a functional style.
  • On Lisp
    Covers fairly advanced topics.
• Planet Lisp: The newspaper of Lisp blogs
• AudioVideo on Lisp topics
• Lisp Gotchas
  
• Highly Opinionated Guide to Lisp by Pascal Costanza
• Searchable online Common Lisp documentation at LispDoc.com
• Subjects:
  • Metaobject Protocol

In-depth

• History of Lisp Museum
  
• Papers on Lisp topics
• Obscure Books
• Comparative studies:
  • "Thinking in Java" book exercises completed in both Lisp and Java
  • DNS Message Decoding: A Case Study Comparing Java and Common Lisp
  • Python for Lisp Programmers
• Other online tutorials:
  • The Common Lisp Cookbook
  • The Complete Idiot's Guide to Common Lisp Packages
  • ELM-ART LISP course
• Understandable source code:
  • Small-CL-src, featuring small, useful code examples
• Cliki resources:
  • CLiki's Online Tutorial page is another excellent source of learning resources
  • CLiki's Education page (which should probably be lifted and transplanted here) has resources on learning Common Lisp
• Translations of lisp terms to other languges:
  • SwedishTranslation
• The old ALU site also has educational resources