ToME Wiki
General Info
Anybody can learn this skill by completing a quest from the Beastmaster (in Lothlorien or Minas Anor). Turning in a specific monster corpse (or meat hacked from one) grants the skill with a multiplier of 0.800.
Strategic Recommendations
For possessors (and possessing loremasters/monks), I find it hard to justify spending points on this skill if you want to make as efficient a character as possible. You will always have more chances to get powerful bodies (if you can kill one GWoP you can probably kill ten).
Most of the really good bodies are uniques, however. You might want Eöl, Maeglin or the Necromancer, and later on Sarko, Kronos or the Tarrasque. And of course the perennial favorite, the Watcher in the Water. You won't be guaranteed any one of these even with a corpse-preservation of 50.000 however.
For summoners, corpse-preservation will help speed up the acquisition of totems, but the same reasoning used for possessors (above) applies: if you can kill one cyclops (or AMHD, or whatever), you can kill another, until one leaves a corpse. Points in corpse-preservation may speed up the game, though.
Melkor worshippers who want to use the Corpse Explosion spell may benefit from having lots of corpses around.
Code-Related Information
Corpse-preservation affects exactly two things as far as I can see.
The chance of monsters leaving a corpse when dispatched is ruled by if ((!force_coin) && (magik(10 + get_skill_scale(SKILL_PRESERVATION, 75))) && (!(m_ptr->mflag & MFLAG_NO_DROP))) Thus, the chance with no points in the skill is 10%, with a maxxed out skill it becomes 85%.
The chance that you get to keep your body when leaving it as a possessor is decided on the line if (magik(25 + get_skill_scale(SKILL_POSSESSION, 25) + get_skill(SKILL_PRESERVATION))) You always have at least a 25% chance of keeping it. Your corpse-preservation can increase this chance by 50%, your possession skill by 25%. Thus you always get to keep your old body if you max out both skills.
Chatter
Not sure about Necromancers yet... same as summoners? Or do they get huge armies from huge piles of corpses?
NeilStevens: Necromancers want corpses for Raise Dead. Once you start getting liches, you get the good old chain summoning going.