Engine rebuild manuals are available online from Amazon.com and TheMotorBookstore.com. Engine repair manuals may also be available at your local auto parts store. Your local library may have some engine rebuilding books available.
You can find engine rebuilding manuals for a particular vehicle. Those manuals have all the specifications for one vehicle, and they sometimes include different model years. General instructions for rebuilding engines are also available.
Engines come in different configurations. General rebuilding manuals may not cover all engines. JacksSmallEngines.com offers engine rebuilding for small engines, such as lawnmowers and chainsaws. AircraftSpruce.com has manuals for rebuilding aircraft engines, though it has a disclaimer saying its manuals are for qualified mechanics.