The subject of "Annabel Lee" is the relationship between the speaker and the titular Annabel Lee, and the ongoing loving feelings the speaker has for her despite the fact that she has died. He claims that, while alive, Annabel Lee lived just to love him and to be loved by him. They were both "child[ren]," he says, both youthful and innocent, and this made their love so pure and even enviable to the angels. One day, however, Annabel Lee caught a chill and died, and her family came and took her body away to bury. Now, the speaker longs for her always, and he even goes to sleep in her tomb alongside her corpse.
Tone describes the author's feelings regarding the subject, and Poe treats the speaker in a rather sympathetic way. He presents the speaker as sincere and despairing as a result of the loss of his love; there does not seem to be any harsh judgment of the speaker, a man who simply cannot go on with his life after his terrible loss.
Mood describes the feelings created within the reader by the text, and so the mood of this poem is sad, certainly, but also somewhat chilling. The speaker does not seem to realize how strange it is that he continues to "lie down by the side" of Annabel Lee, even in death, creating dramatic irony: we know that his behavior is unhealthy and creepy, but he does not seem to realize it.