SICP
"Structure and Interpretation of Computer Programs" is a very
well-known and well-respected computer science text, used
(among other places) at MIT for introductory programming
classes. It's based around LISP/Scheme. An online version of
the full text of the book was made available as part of the (now
defunct) ArsDigita University. The download site appears to be
either dead or overloaded, but thanks to the
Internet Wayback Machine, the book
is still available.