It's almost like a real car when to get a tune up. (Me personally) I normally change a typical O.S. A-3 glowplug after each gallon of fuel. There is no way to tell by looking at a glowplug of how good it is no matter how bright it looks either, unless the engine is running funny. There are glowplug testers you can get that can tell you how good they are but really isn't worth it. If you want a good quality glowplug, you should get an O.S. #8 Glowplug. Again I'd change it after every gallon of fuel used to be safe and get top performance. But this #8 will make a difference by lasting longer and running hotter for faster speeds. Remember though when breaking in an engine you may go through like 2 or 3 glowplugs before the engine is broken in. This doesn't always happen, but can. So start out with the cheap ones (OS A-3) if the engine is new.

For as gearing goes, depends on if you want faster take offs, or faster top end speeds. You can start by getting 2-speed transmission. Works just like a real transmission in a car. Or you can simply get bigger clutch bell and the smallest fly wheel possible. You can gain 600rpm easily just by switching to a smaller fly wheel. A bigger clutch bell may slow down your take off a little but will give it more top end speed. 2-speed transmission will be my number one suggestion because of it gives you both a fast take off and even faster top end.

And no I don't know of an good books to study this stuff on, but information just comes over the years and from expierneced hobbyists. And if you have any other quesitons. ASK! I'd be more than happy to help you out.