She might be very angry. That novel could be very important to her, perhaps it's a symbol of her first love, and you stealing it is like taking away something very precious.
You should probably consider returning it. Stealing is not right, and it belongs to the heroine. If you return it and apologize sincerely, she might forgive you.
The main spoiler is that the gift exchanged between the leads holds a special meaning that changes the course of their relationship. Also, there's a surprise visit from a long-lost relative that complicates matters but ultimately brings them closer together.
Well, I can't give you specific spoilers. But usually, it might involve unexpected plot twists or revelations about the relationship between the main characters.
I stole the roses, but I love you, it's true.
I stole the rose but I truly love you
This line suggests that one person is saying they committed a sin by stealing the other person's property (the rose) but they still deeply love the other person and would never want to let them down It shows a level of integrity and commitment that is not seen in many relationships today.
In the novel, it turns out that one of the supporting characters has a hidden agenda that impacts the whole story. Also, there's a big twist about the protagonist's past that changes everything. But I won't say more to avoid spoiling it completely for you!