The title of this post is a trick question. I have no idea and I don't know if Matthew Pearl does either. Maybe no one did. Pearl has written a book though about a guy who tries to uncover the true circumstances behind Poe's mysterious death.
The Poe Shadow won't be released until May 23rd, but you can pre-order it now on Amazon.com. It follows a Poe fan's quest to learn what really happened to the legendary writer with the aid (or hindrance) of a couple of guys who claim to be the inspiration for Poe's best-known character, Inspector Dupin. The subject matter and the nineteenth century setting make this one a Must Read for me.
Incidentally, Pearl has another literary mystery, The Dante Club, about a group of Dante scholars who attempt to solve a series of murders inspired by Dante's descriptions of punishments in Hell. Not being much of a Dante reader, I'm less drawn to this one, but it's an interesting idea. Just not as interesting as a hypothetical solution to a real-life mystery like Poe's death.
(Thanks to Bookgasm for pointing this one out.)