There are a couple Forth books that really break the mold when it comes to technical manuals. I would recommend "Starting Forth" by Leo Brodie as a good introduction to the languages. Leo's style is casual and fun to read. The cutesy pictures are a bit over the top, but it works.
I also liked "Thinking Forth" by the same author. TF throws a splash of cold water on the traditional engineering approach to software development. It is a bit anti-structured, a bit anti-OO, but in a way that turns your way of thinking around. Its hard to describe. The absolute best part of the book are the captioned illustrations. Priceless.
Both books are out of print, but used copies are available on the net.