Factory service manuals, Alldata or Mitchell is the way to go.
That said, I have a Haynes or Chilton manual for every vehicle I own. Some of them came with the vehicle when I bought it, some I bought myself. I learned long ago that they're only really good for torque specs and sopping up spilled oil. Chiltons is a little more in-depth, but lacks a lot of pictures. Haynes has more pictures but less detail. And the wiring diagrams in either flat out suck. I spent some time studying to be an electrical engineer and the diagrams look to me like they were drawn by a kid with a crayon - half the stuff you could stand to know is just not there.
Mostly I rely on my knowledge I've gained over time, this site, and when I need a good wiring diagram, I'm on friendly terms with a few garages to get a peek at Alldata or Mitchell. I'd love to have my own subscription to one of those two but...