Now in it's Third Edition (2004), this enduring work is the only book to fully explore not just the semantics of the Forth programming language, but also the philosophy and style that is used by its most successful practitioners. Interestingly, this philosophy applies well to programming practice in any language.

Leo Brodie (http://home.earthlink.net/~lbrodie/) wrote Thinking Forth in 1984. Since the book went out of print now the second time (due to FIG folding up - FIG did the reprint), he allowed to publish Thinking Forth as electronic book under a Creative Commons license (attribute, non-commercial, share-alike)

http://thinking-forth.sourceforge.net/