I just read this one. Piano no Mori or its other name The Perfect World of Kai? Piano no Mori is an irregular ongoing manga by Makoto Isshiki who won a Grand Prize for this manga at the 2008 Japan Media Arts Festival. And if you want to know, Makoto Isshiki was first inspired to write Piano no Mori when he watched a documentary showing Stanislav Bunin winning the International Frederick Chopin Piano Competition in 1985.

The series was even adapted to a animated feature film by director Masayuki Kojima and featured performances by the renowned pianist Vladimir Ashkenazy. The movie adaptation debuted in 9th place at the Japanese box office the week it came out, which is unusually high for a non-franchise animated film. By the end of the year, it had grossed the equivalent of $1,555,297,  with ranking 119 on the overall yearly box office chart for Japan. The film was nominated for the 2008 Japan Academy Prize for Animation of the Year. It was also nominated in the Theatrical Film category at the 2008 Annecy International Animated Film Festival. The film made its Canadian / North American premiere at the 2008 Waterloo Festival for Animated Cinema.