There are a few books located in the actual bases of of the enemy factions. Example the books pact recruitment, Conquer Cyrodil for the Pact, Those who stood at Chalamin Keep, Updated instructions from Dortene, Information request from Emeric, A Warning from Zimmeron and Chastisement from Zimmeron. There was some google talk of them being at the temples or in Hew's Bane?
Any direction would be welcome