"What are the most widely used methods of lead bullet tempering?"
Never bothered with it. For loads in the 700fps range, I shoot straight wheelweight material - zip for any hardening ingredients.
"For lead, does tempering = hardening?"
Yup = surface hardening. Don't know to what depth.
"How much of this hardening does resizing remove? (i.e., how much of the bullet is typically removed by the sizing die?)"
You shouldn't be removing any material by sizing. Sizing only swages the material into round. & BTW, recommended sizing should only be about .001" - just enough to round out the bullet & never enough to remove any mat'l.
"Recommended literature for my education on this (besides Lyman, of course)?"
Other good books out there but if you've got Lyman, you've got the guys that wrote the book.
I cast for .38/.357, .308/30-06/.309JDJ (& have for .44). Straight wheelweights for ~700fps & Lyman's #2 mix for everything up to ~1700fps. Use a good lube (get the heater plate & use hard lube if you can - the regular stuff is so sticky!)
Never a single leading problem w/above & have turned in some very accurate 100yd targets with the .30 cal shooters.