In *Kingdom Come: Deliverance 2*, missing certain side tasks can lead to permanently failing them, and Rosa's Book quest is one such task you don't want to overlook. To embark on this quest, you first need to complete "Via Argentum" and begin "Taking French Leave." After aiding the Ruthard family in dealing with Vavak—either through force or persuasion—you'll be instructed to speak with Rosa. She'll reveal a secret passage into Maleshov that you can use to rescue Hans, and in the same conversation, she'll ask you to retrieve her book while you're there. Make sure to ask her about the specific book and promise to bring it back to initiate "Rosa's Book."

Remember, you can only complete this quest during "Taking French Leave." If you leave Maleshov with Hans without securing the book, you'll fail the quest and miss out on the chance 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 held at the top. Use the side stairs to enter the building discreetly. Inside, you'll find another set of stairs and a small kitchen nearby. Use the kitchen to hide until the guards on the upper floor leave the tower.

Quietly ascend the stairs to a room where Lady Ofka is seated, facing the fireplace. You can sneak up behind her and knock her out. Retrieve the keys from her body, the podium in her room, and a sword from the chest. In this room, you'll also find Rosa's book. It's positioned to the right of the fireplace in the window alcove. Once you pick it up, your objective updates to return the book to Rosa.

Before leaving, enter the adjacent room and take another sword from the chest there. This will save you time later as you'll need two weapons for Hans and his surprise guest. Proceed to the top level and unlock the door on the left to trigger a cutscene.

Following the cutscene, continue with "Taking French Leave." Although Hans might not want to use the passageway, you can progress towards romancing him by choosing the dialogue option with a heart icon, provided you also selected the heart choice in Trosky. Next, stealthily navigate to the stables on the north side of Maleshov, knocking out and hiding guards as necessary. After clearing a path, signal Hans to ensure his safe passage to the stables.

Once you open the gate, you'll trigger another cutscene, concluding the quest. After it ends, return to Kuttenberg City and stop by Ruthard Palace to deliver Rosa's much-coveted book in *Kingdom Come: Deliverance 2*.