In Dawnguard, Marethi says that he is tired of the politics he experienced in House Dres. In A Game at Dinner, the anonymous author claims that he was greatly disturbed by Helseth's game, and requested that Dhaunayne relieve him of his service to House Dres.
I say they're the same person, and Bethesda just made an awesome reference to an awesome book through Garan Marethi.