This tutorial (or maybe it should rather be calles manual ?) should help you get started with GRASS. It gives basic explanations of how GRASS functions and links to the relevant man pages and other documentation.
Several chapters in this tutorial contain content taken from the english translation of Markus Neteler's GRASS Handbook, (c) 1996-2001, Markus Neteler. I have not included explicit references each time I've used excerpts, but it is the most important source of text in these pages.