Also, who is your favorite?
Waughin Jarth, to both questions. For reference, UESP conveniently has a page organizing all of the http://uesp.net/wiki/Lore:Books_by_Author. Jarth has seven separate series, but counting each book in each series separately he's got 25 books to his name. (stray thought, I hate that the plural of series is series)
In theory, he would have the most, but it's more like a single book split into 36 pieces. Most of Waughin's stuff is the same way, but he still has the most works present after lumping volumes into single works. I'm also pretty sure it's easy to debate whether or not Vivec actually wrote the sermons himself.