I couldn't find the thread, but somewhere someone claimed to have received a spellbook called Conjurer's Grimoire from the graveyard spawns. I just got one, so I went ahead and took a pic. It looks like the skeletal liches drop an undead slayer talisman and this spellbook. I have only received vampire bats from the black things that also spawn in doom. PS. I got this spellbook after about 10 or so skeletal liches. I suspect it's not too terribly rare, but rather uncommon.