Blades Quest 4: Vivec
Caius is all about these information. Your next quest involves getting information out of three more people (Addhiranirr, Huleeya and Mehra Milo) — all in Vivec.
If you're not a fan of exploration (or of legwork), the Silt Strider will take you to Vivec. (It's probably easiest to go for Huleeya first, since he's in the Foreign Quarter which is nearest to the Strider.)
You'll find Huleeya in the Lower Waistworks of the Foreign Quarter (each area in Vivec consists roughly of a Plaza on top, Upper and Lower waistworks in the middle, and Canalworks and Underworks on the bottom.) On the east side is the Black Shalk bar, where Hueeya is. Guess what, it's not that easy. Talk to him, and learn of his troubles getting to his friend's store. Ask him more to learn more about the situation. Then, go talk to the fools that are causing his grief and bribe or admire them to get them to like you. Then, ask them about the "filthy lizard". When you're successfu, they'll say they don't care. Then, return to Huleeya and escort him to the bookstore. Talk to him about the Nerevarine cult to get the notes for Caius.
Addhirnirr is located in the St. Olms canton of Vivec in the Underworks (you need to cross through the Waistworks and down through Canalworks to get there.) You'll see a trap door to the St. Olms Underworks. You'll find Addhiranirr in the northeast area. When you talk to her, she mentions her troubles. Ask her about the Census and Excise agent to continue. She'll ask you to get rid of this man, Duvianus Platorius. You'll find him in the Waistworks. Tell him that Addhirnirr went to the "mainland". Then, return to Addhirnirr and ask abou thte Sixth House Cult. She'll give you what you need.
Finally, Mehra Milo can be found in the Hall of Wisdom in the Temple canton. If you're coming from the St. Olms region, enter the door straight ahead from the bridge, and go down the hallway half way and follow a ramp to the library. Mehra is in there. Talk to her — she'll bring you to the back. When you talk to her again, ask her about the cult and the "Progress of Truth". She asks you to get a copy of the book for Caius. You have two options: steal one from the southwest corner of the library (if you have the skill), or buy one from Jobasha's Rare Bookstore.
Finally make your way back to Balmora and give Caius everything you picked up. He'll promote you to "Journeyman".