Practical Common Lisp (Books for Professionals by Professionals)

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Practical Common Lisp is an introductory book on Common Lisp by Peter Seibel which intersperses "practical" chapters along with a fairly complete introduction to the language. In the practical chapters Seibel develops various pieces of software such as a unit testing framework, a library for parsing ID3 tags, a spam filter, and a SHOUTcast server.
The complete text is available online.