In *Kingdom Come: Deliverance 2*, Rosa's book side quest is easy to miss, but it's crucial for those aiming to romance Rosa. To unlock this side quest, you first need to complete the "Via Argentum" quest and begin "Taking French Leave." After assisting the Ruthard family with Vavak, whether through force or persuasion, you'll be directed to speak with Rosa. She'll reveal a secret passage into Maleshov to rescue Hans and simultaneously ask you to retrieve her book. To start "Rosa’s Book," inquire about the book and promise to bring it back to her.
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Remember, you can only complete this quest during "Taking French Leave." Failing to find the book before leaving with Hans will result in the quest failing, and you'll miss the opportunity to romance Rosa. Fortunately, the book is conveniently located on your path to Hans' room.
Upon entering Maleshov, head to the largest tower where Hans is located at the top. Use the side stairs to reach the entrance, then sneak inside. Near the entrance, you'll find another set of stairs and a small kitchen. Wait in the kitchen until the guards upstairs leave the tower.
Screenshot by The Escapist
Next, quietly ascend the stairs to find a room with Lady Ofka sitting by the fireplace. Sneak behind her, knock her out, and take the keys from her body and the podium, as well as a sword from the chest in her room. Here, you'll also find Rosa's book. Look to the right of the fireplace to spot a red book in the window alcove. Pick it up, and your objective will update to return the book to Rosa.
Screenshot by The Escapist
Before leaving, enter the adjacent room to grab another sword from the chest, which will save you time when meeting Hans, as you'll need two weapons for him and his surprise guest. Then, head to the top level and unlock the left door to trigger a cutscene.
After the cutscene, continue with "Taking French Leave." Although Hans refuses to use the passageway, you can progress towards romancing him by choosing the heart dialogue option, provided you selected the same option in Trosky. Sneak to the north side stables of Maleshov, knocking out and hiding guards as necessary. Clear a path, signal Hans, and ensure he reaches the stables safely.
Open the gate to initiate another cutscene, concluding the quest. Afterward, return to Kuttenberg City and stop at Ruthard Palace to deliver Rosa's book in *Kingdom Come: Deliverance 2*, fulfilling her request and advancing your relationship with her.