If you want to know about the very early history of GM including the beginnings of Pontiac and Oakland read this:
Sloan, Alfred P., Jr. My Years with General Motors, 1963
It's a bit dry and tedious in parts due to the presentation of financial information, production numbers, and administrative policy, but there is lots of info on the early inner workings of GM and interesting tidbits. For example, Walter P. Chrysler and Charles Nash worked for Buick, Chevrolet once owned GM, Buick was the original product, etc.