Overview
A Tome Entombed is a secondary quest in The Witcher 3: Wild Hunt, hidden deep in the sewers of Novigrad. You can get there through a tunnel in the middle of The Bits, or through the tower door by the Tretogor Gate signpost and down the ladders. Look for a weak wall on the left before a gated archway. You can also stumble on it while chasing Duke in A Dangerous Game.
The professor and the coffin
Past two weak walls, a man is struggling with a sealed sarcophagus. He is Professor Vairmont Jonne, convinced it contains an elven manuscript. Geralt pushes the lid aside. Inside is a sleeping vampire, who notes that it is not the right year yet, tells them rudely to go away, and shuts the lid.
The professor wants to try again, since he did not get a proper look. Geralt can refuse, but then the quest stays open in the journal until he opens the coffin a second time.
The second time
This time the vampire is furious and takes his true form, a katakan. After the fight, the professor remarks that Geralt never gave him a chance to ask about the manuscript, shrugs and leaves. Some loot waits up the stairs behind the coffin.