This may be my new favourite book. It has 400+ pages, but I read it in 3 days. It was captivating and I couldn't put it down.
The book is split into 2 timelines. First timeline is set in "the present" and follows Ruth and her husband Oliver, who live on a small island that belongs to Canada. The second timeline is a journal from a 16yr old Tokyo girl called Nao. One day, while Ruth was on the beach she found a hello kitty box that washed up on the shore. The box contains letters, a book by Marcel Proust and a watch. It turned out that the book is actually Nao's journal. Thus Ruth begins reading Nao's journal together with Oliver and overtime grows more and more invested in Nao's life and other people in her life. That is the basic plotline. But through Nao and Ruth's lives, this book explores the neverending topic of the human condition. It also deals with highschool bullying, mental illness, the war, family relations and much more with a hint of magical realism. I thought it was wonderful. 
I loved how Ozeki incorporated all the elements of physics, such as the concept of time and quantum mechanics. It wasn't difficult to understand, nor did it feel out of place. Just the perfect amount of exolanations for the story to take a certain direction. Since one half of the book is set in Japan, we get to explore and see different places in Japan: the lively loud urban places and quiet calming countryside (Studio Ghibli vibes); their culture, language and bits of history and traditions. I especially enjoyed that.
Regarding Nao's story, I must say it was difficult to get through. Nao was born in America and when she was a teen, her father lost his job at an IT company and they had to move to Japan. She did not handle this transition well and her classmates bullied her every single day because she was different. She barely spoke Japanese, didn't know much about their way of life or mentality, so it was tough for her to get used to it.

One day, it escaled to the point of attemped rape in the bathroom of the school. The classmates filmed it and posted it online. They also took of her undies and auctioned them on the internet.

That was truly horrific. Besides her issues at school, her dad had been battling depression and suicidal thoughts ever since they returned from America.

He attemped to end his life twice. The second time was after he found out about the auction. He tried to win the auction so he could spare his daughter the embarrassment and trauma, but failed to do so.

A lot of messed up things happen in this book, but it doesn't feel romanticised nor glorified. It is told in a way for us readers to see the sad reality of many Japanese highschoolers.
I loved this book and at this point in time I'm giving it 5 stars because I think it all makes sense to me (the plot and how it was written and all the little quirks). Do I recommend it? I don't know. Make sure to check the trigger warnings beforehand if you wanna read.