You can read the description of the novel below. (Thanks to KH13 team member @Mio-chan for the translations!)
The first volume of the novel, which released in Japan last March, can be purchased here.
UPDATE [Jun 25, 2019]: Square Enix have revealed the cover for the Game Novels Kingdom Hearts III Vol. 2 New Seven Hearts! If purchased from the Square Enix e-Store (Japan), the book will also come with a postcard showcasing the same cover art! You can see the cover art below.