As Bill suggested, Rob English's oldGMCtrucks.com website has a wealth of information. You might also want to consider joining the the oldGMCtrucks.com discussion boards.

If you are interested in reading a history of GMC trucks, GMC: The First 100 Years. It has mistakes in it but it is a good historical run-through.

Most likely, you'll want to obtain GMC Maintenance Manual.

There is a "Book Store" forum on the oldGMCtrucks discussion board. One member has put together a nice CD-ROM library of useful manuals.