It seems as if he is grieving, trying to forget a loss of a person. His mental state seems to be in that place between sleep and wakefulness as he lets us know it is late at night, he is nodding, and nearly napping and it seems he's into a book (one of the best ways many of us fall asleep). He is in front of a fire, likely very warm, and his body quite comfortable in an easy chair although his mind is not at all at ease.
You mention the beginning of the poem. The whole poem is certainly about trying to forget someone who has passed, but the beginning does use the word "forgotten" in reference to lore. Lore means ancient literature or books. He may have been reading something that many have forgotten. His intention in reading was to help him forget his sorrow, sorrow for the loss of someone named Lenore.