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Did you find a solution in the end? Exact same thing has happened to me - I completed Circus of Cheerful Slaughter but Shalidor hasn't appeared, while Valaste in Malabal Tor says I need to collect more books. I don't think I have a quest log entry, so I can't even abandon the quest to restart.
Each Mages Guild quest requires the next Mages Guild rank. So for example, the quest after Circus of Cheerful Slaughter requires Mages Guild Rank 4. As Valaste says, you need to collect more Lorebooks if you have not yet reached the required rank to start the next quest. You can also complete the Mages Guild daily quests in your alliance capital to increase your rank.