In *Kingdom Come: Deliverance 2*, missing out on some side tasks can be frustrating, especially when they can't be revisited. One such task is finding Rosa's book, which is crucial for those aiming to deepen their relationship with her. To unlock the "Rosa's Book" side quest, you first need to complete the "Via Argentum" quest and begin "Taking French Leave." After aiding the Ruthard family with Vavak, either through persuasion or force, you'll be directed to speak with Rosa. She'll mention a secret passage into Maleshov for Hans' rescue and then ask you to retrieve a book for her. Make sure to inquire about the specific book and promise to bring it to her to officially start the "Rosa's Book" quest.

It's important to note that this quest must be completed during "Taking French Leave." If you proceed to leave with Hans without finding the book, you'll fail the quest and miss the opportunity to romance Rosa. Fortunately, the book is conveniently located along your path to Hans' room.

Upon entering Maleshov, head to the largest tower where Hans is located at the top. Use the stairs on the side of the building to enter discreetly. Once inside, you'll find another set of stairs near a small kitchen. Wait in the kitchen until the guards upstairs leave the tower.

Sneak up the stairs to a room where Lady Ofka is seated, facing the fireplace. You can sneak up behind her and knock her out. Take the keys from her, the keys from the podium, and the sword from the chest in her room. In this room, you'll find Rosa's book to the right of the fireplace, sitting in the window alcove. Pick it up to update your objective to return the book to Rosa.

While in the area, grab another sword from the chest in the adjacent room to save time later when meeting Hans, as you'll need two weapons for him and his surprise guest. Then, head to the top level and unlock the door on the left to trigger a cutscene.

After the cutscene, continue with "Taking French Leave." Although Hans won't use the passageway, you can advance towards romancing him by selecting the heart dialogue option, provided you chose the same in Trosky. Sneak to the north side stables of Maleshov, knocking out and hiding guards as necessary. Clear a path for Hans and ensure he reaches the stables safely.

Open the gate to initiate another cutscene, concluding the quest. Once it's over, return to Kuttenberg City and stop by Ruthard Palace to deliver Rosa's much-desired book in *Kingdom Come: Deliverance 2*.