Haynes Manuals are a good basic knowledge of engines, these manuals below are specific if you want a real good service manual. Most of us use the newsgroups to get PDF factory manuals but these options below if you want hard copied manuals. It can get expensive over time when buying a lot of service manuals. I stick with honda,brigs and stratton or generac for generators and engines. Honda is usually a little to much for my blood. Engines aren't to bad to rebuild it's the parts that kills you. Even in the third world countries when traveling some of the most simplest things are near imposable to fix with out tools or parts and a welding torch is a must have when parts are scarce.
I'm not sure what type of engine your after but maybe this will help a little.


http://www.repairmanual.com/small_engines/

http://www.jackssmallengines.com/serbs.cfm

http://www.amazon.com/Small-Engine-Repair-Haynes-Manuals/dp/1850106665