As far as qaulity books (not chiltons, Haynes, etc...) I have a shop manual for a 1990 Lincoln Mark VII. It is excellent. I dont have the Lincoln anymore, but it comes in very handy with the stang, as the drivetrain is the same with both cars, and the manual has excellent and in depth coverage of just about every system on the car.
I buy from
www.helminc.com
They break it down into several manuals for each car model. The maintenance manual is the biggest book and costs the most, around $80.00.
Then they usually have more specialized information in separate books, such as a book for Emmisions systems, and a separate book for "electrical and vacuum troubleshooting". They are usually around $25.00 each.
For the mustang, I only have the "electrical and vacuum troubleshooting" book. It is basically just all the wiring diagrams of the car broken down into different systems, with color codes to identify and locate a particular wire anywhere on the car, which comes in handy. Might be more useful on an older car that has more electrical problems than a newer, more reliable model. i get more use out of the maintenance manual.
I guess "Mitchells" is the standard that shops use, like PKRWUDS, and they have time allotments for each job to use to charge customers the labor rate. I dont know where to buy these books, but they are more for garages than individuals.