BNielsen
I'm Awesome
While Haynes is decent & pretty much all that's available nowadays, I prefer Chilton's (yes I know that 2 one point they were being made by the same company), I find them to be more informative. Clyne's were even better, but I don't believe they were out business before the GMT400's (yes, I just dated myself). If you can swing the dough, get a set of Helm's (factory style manuals).
I've got all three, I've got a Haynes I keep in the truck with my scanner, a Chilton with some notes written in it, and two fat GM Tech manuals; I was weary about spending the money on the shop books but damn is it worth it.
150 is nothing for a 4l80e, even not well taken care of.. hopefully it isn't hurt. Honestly if it is, and you're on a budget, I'd go after a low mileage takeout.
Also for performance a tune is the best bang for the buck, exhaust is also high up there.
This, something as simple as a Black Bear tune will surprise you with the difference it makes, I wish I would've just gone with a more aggressive tune when I bought my ECM from them, free retunes for life, all you gotta do is pay shipping and they'll make any changes you want.