A very good introduction to forth is at http://en.wikipedia.org/wiki/Forth_language.
Starting Forth is a great book by Leo Brodie for the beginning programmer. Marcel Hendrix wrote an online adaption here: http://home.iae.nl/users/mhx/sf.html
Wonderful book about how to be a great forth programmer, also by Leo Brodie: Thinking Forth
Another good (and brief) introduction can be found at http://www.ece.cmu.edu/~koopman/forth/hopl.html.
ANS Forth tutorial: http://www.xs4all.nl/~thebeez/ForthPrimer/Forth_primer.html
ANS word reference (alphabetic list of words with links to their definition in the draft standard): http://ficl.sourceforge.net/dpans/dpansf.htm