For general reference material on GIS, try a very good technical bookstore (in many cases these are campus bookstores at schools with good GIS programs or top-notch technical or general bookstores - you know that ones are near you..), or the following URL for the CyberInstitute Short Course on Geographic Information Systems (convened by myself):
For a good collection of references on GRASS, try this procedure, to load up on reference goodies from USA/CERL:
ftp moon.cecer.army.mil login: anonymous password: your email address cd pub/grass/grass4.1/outgoing image get grassman.ps.Z (or grassman.txt.Z, or grassman.wp.Z) cd ../documents/programmer/postscript image get progman.ps.Z cd ../../user/postscript image get refman.ps.Z cd ../.. image get installGuide.ps.Z bye uncompress grassman.ps.Z uncompress progman.ps.Z uncompress refman.ps.Z uncompress installGuide.ps.Z lpr *.ps (or whatever is appropriate for your environment)
installGuide => The GRASS Installation Guide (you need this to compile GRASS source code)
grassman => The GRASS Beginner's Manual (intro to GRASS)
refman => The GRASS User's Reference Manual (function guide)
progman => The GRASS Programmer's Manual (and administrator's guide - this is valuable for info about data formats, etc.)
Browse around the ftp site noted just above, and you may find more stuff of interest. Particularly in the pub/grass/grass4.1/documents directory, there are tutorials on advanced GRASS functions such as r.mapcalc (think of this as math applied to raster arrays), r.combine and r.weight (think of this as how to combine spatial submodels into one type of model), and others.