If Only

Ian Wyndham (Nicholls) is an English businessman who lives with his American musician girlfriend, Samantha Andrews (Hewitt) in London. Taking us through a day in Ian and Sam's life, the film opens by showing different events such as Sam getting burnt on a kettle, Ian's watch breaking, Sam getting Coca-Cola on her and Ian being interrupted by Sam during an important meeting at work.
Sam gets angry at Ian as he always treats her as his second priority and tells him that she just wants him to love her. Ian makes one last attempt to reconcile with Sam. An angry Sam gets into a taxi and asks Ian whether he is coming in or not. The taxi meets with an accident and Sam dies. A heartbroken Ian goes back to his apartment, finds Sam's notebook and opens it, finding a song she was working on. He falls asleep clutching the notebook close to him.
When he gets up the next morning, he hears Sam talking and sees her in the bed right next to him. He is scared at first but after the initial confusion, Ian comes to the conclusion that the previous day must have been a dream. Ian feels happy to have Sam by his side.
The events of the next day are similar to those he already experienced in the dream, although occurring at different times and in different ways. Ian is sure that he had a premonition and that the end of the day will be same and Sam will die. Sam gets Coca-Cola spilt on her. But Ian's watch remains in working order. After talking to the taxi driver he realizes that the driver is the man from the dream who asks Ian to love Sam.
After convincing her to come with him, they travel to Ian's rural home town. In the town below, the two have drinks and Ian tells Sam about his father, who lost his beloved job before becoming an alcoholic and dying sometime later. The two travel back to London and Ian takes Sam on the London Eye as another surprise. They then travel back to their apartment and Ian takes a page from her notebook and brings it to a nearby photocopying shop while Sam travels to her concert with her violin. Before the show begins Ian gives the photocopied pages to an organizer. After the performance in which Sam is a violinist, the organiser announces Sam's name as the next performer. A nervous Sam proceeds onto the stage as the orchestra begins to play the song printed on Ian's photocopied sheets. She sings the song she wrote for Ian in her notebook and the crowd burst into applause at her performance.
Instead of going to Tantra, the restaurant that Ian had made reservations for, they go to a more intimate restaurant that Sam chooses, but Ian loves it as well. Ian gifts her a bracelet with meaningful charms. They have a nice time. After the dinner, as both of them are standing outside of Tantra, where they ate in the dream, Ian tells Sam why he loves her. As the taxi pulls over and Sam gets in, she asks him again whether he is getting or not. This time Ian gets in and sees that the cab driver is the same again. As the clock strikes 11, the taxi meets with an accident.
The same scene is repeated again, when Sam's friend is shown running in the hospital corridor. Only this time, unlike the dream, its Sam who is sitting and crying instead of Ian, implying that Ian died in the crash.

Question: to whom the notebook belonged to?
Answer: Sam's