Title: 告白 
Author: 湊かなえ Kanae Minato
I must say this is an intriguing book to read. It may be one of those talked about book in Japan but without the stellar movie, it would not have garner so much attention. And if not for the soundtrack, the movie would not have been as successful. At least, that's my own take on it.
A psychological thriller needs the tension of emotional strings to tug at our sense of involvement. And one cannot help but get involved with the various protagonists in the book and movie.
The film has been selected as the Japanese entry for the Best Foreign Language Film for the 93rd Oscars.
If you do not read the book, watch the movie. If you can - do both!