Do you need schematics for just the motherboard? The PC AT ref is a fairly fat book with descriptions for several peripherals. A huge amount of information, much of it useless.
Come to think of it, I don't know if anyone's ever scanned the Options and Adapters series of books. That'd be a real job.
I wonder if the suits at IBM are still obsessive about copyright issues for manuals on products they haven't manufactured for almost 20 years.