How to unlock: Speak with Falinda (M3.10).
To unlock this quests, you have to talk to Falinda, the bookseller. The woman can be found in the Circus Minor district (M3.10). During the conversation, you will learn that one of her books is missing. She will then ask you to retrieve it.
At this point, there's no point in looking for the book since you'll be able to obtain it in the course of The Cold, Calculating Jack quest, in the secret sanctuary. Once there, head west to the mirror located in the center of the room where you will find two chests placed next to one another (M11.11). Interact with them and take the missing book out of them.
When you're back from your trip to the sanctuary, go to the woman (M3.10) and initiate a dialogue. You will be then able to give the book back to her (you will receive 25XP). You're going to have 5 dialogue options to choose from, 4 of which grant you a book as a reward. It has no impact on the plot so take your pick (you can sell each of the books received to any merchant). The quest is over.