The idiot drove indirectly himself to insanity
He was reading "an ancient tome of forgotten lore" (i.e., a Tome of Eldritch Lore) and summoned an evil spirit that consumed his sanity. The raven was a sluagh
For those of you unfamiliar with Celtic Mythology, the sluagh are damned souls who take the form of Creepy Crows. They can be summoned by saying their name or passing the Despair Event Horizon. So clearly it devoured Lenore's soul and so the narrator would summon it with his despair and feast some more. The volumes of forgotten lore are books of necromancy
The narrator is searching for a way to bring Lenore Back from the Dead.