Structure and Interpretation of Computer Programs

You come in with a mind full of mush,
and you leave thinking like a computer scientist.

The famous Wizard Book by Harold Abelson and Gerald Jay Sussman with Julie Sussman. The text of this book is available for free:

HTML (unofficial)
PDF with vector graphics

A series of video lectures taught by two of the authors is also available for free, filmed at Hewlett-Packard in July 1986.

Although the book uses Scheme, it is certainly one of the great lisp books.