Comment by yawl

Modern Compiler Implementation by Andrew-Appel is a very good introduction book for people to get started on doing a compiler.

And if you want to be more pragmatic, here is a shorter one: http://www.cl.cam.ac.uk/teaching/2004/CompConstr/NEJ/report....

The dragon book is a MUST for compiler researcher and people who are serious on compiler, but it is hard to follow, and I do not recommend it as your first compiler book.

For ruby, you can take a look of the xruby compiler I did: http://xruby.googlecode.com (google code's server has some problem now, you may have to wait for a while)