If you start to get serious about using Lisp, you’re likely to want to buy some books to have on the shelf. I’ve got three: Practical Common Lisp by Peter Siebel, ANSI Common Lisp by Paul Graham, and The Art of the Metaobject Protocol by Gregot Kiczales, Jim des Rivieres, and Daniel G. Bobrow.
For people who want to try out the language before committing money to the attempt, though, there are online sources. Notably, Practical Common Lisp has been made available online by the author. It can be found here.
Another very useful online resource is the Common Lisp HyperSpec, which I mentioned earlier in this series. It provides good syntax descriptions and examples of all of the standard-defined features of Common Lisp.
You may also want to check out the advice outlined on this page.
A well-appointed public library will usually have a few books on Lisp programming on the shelf.