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.
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!
The most obvious theme is lost love, of course. It's about the pain and longing that comes with losing someone you love. Another theme could be regret, thinking about things that could have been done differently in the relationship.
Well, one major spoiler is a surprising plot twist involving the main character's true identity. But I won't go into too much detail to ruin it for you!