In *Kingdom Come: Deliverance 2*, a crucial side quest, easily missed and impossible to replay, involves retrieving Rosa's book. This guide ensures you won't miss out.
Before Rosa requests the book, complete "Via Argentum" and begin "Taking French Leave." After resolving the Vavak situation with the Ruthard family (through either force or diplomacy), speak with Rosa to learn about a secret Maleshov passage for rescuing Hans. She'll mention the book, and asking about it initiates the "Rosa's Book" quest. This quest is only available during "Taking French Leave." Failing to retrieve the book before leaving with Hans results in a failed quest and prevents a potential romance with Rosa.

Upon entering Maleshov, Hans is located in the tallest tower. Use the external stairs to access the tower's interior. Inside, you'll find another stairwell and a small kitchen. Wait in the kitchen until the guards above leave the tower.

Stealthily ascend the stairs to a room occupied by Lady Ofka. Knock her out, and collect the keys from her person, the podium, and the sword from the chest. Rosa's book is in this room—a red book in the window alcove to the right of the fireplace. Grab it, and your objective updates to returning it to Rosa.

While there, take the sword from the chest in the adjacent room. This saves time later when equipping Hans and his unexpected guest. Proceed to the top floor, unlock the left-hand door, and trigger a cutscene.
Following the cutscene, continue "Taking French Leave." While Hans may initially refuse the passage, choosing the heart dialogue option (assuming you also selected the heart option in Trosky) advances a potential romance. Stealthily navigate to the stables north of Maleshov, incapacitating and hiding guards. Once the path is clear, signal Hans to ensure his safe arrival at the stables.
A final cutscene concludes the quest. Afterwards, return to Kuttenberg City and deliver Rosa's book at Ruthard Palace, completing the quest.