Hasphat Antabolis, in the Balmora Fighters Guild, has a collection of notes for the Nerevarine to deliver to Caius Cosades. He will give them these notes once they get him a Dwemer puzzle box from some Dwemer Ruins (part of the main quest). Dorisa Darvel sells none of the books he lists in the notes.
[The following are notes prepared by Hasphat Antabolis for Caius Cosades.]
House Dagoth is an extinct Great House. In the wake of the ancient Battle of Red Mountain, its leadership was revealed to have plotted treason, and was discredited. Many of House Dagoth died defending the House; those survivors who were faithful to the Great Council were redistributed among the other houses. The Temple says the ancient, legendary evil beings that dwell beneath Red Mountain in the Dagoth Ur region are the original leaders of this extinct house, sustained by some powerful, evil sorceries.
These books include references to the Sixth House and its destruction. The bookseller Dorisa Darvel over in the Commercial District might have copies.