I've got to agree with this. Writing Lexers/Parsers/Interpreters is some of the most fun I've had programming. When your program can run other programs, that's a huge thrill.

Read EoPL, it's awesome (and free) and LiSP is awesome but expensive. In fact almost every Lisp related textbook seems to have several lisp interpreters in it.

edit: As noted below EoPL is apparently not free, and it seems that I've inadvertently pirated it from somewhere.

http://www.cs.indiana.edu/eopl/

http://tinyurl.com/lispinsmallpieces

I'm working on a toy (for now) lisp in javascript at the moment and it's profoundly unimpressive but has been hugely fun and educational for me.