To Every You I've Loved Before Novel was written by Yomoji Otono.

In a world where people regularly travel between slightly different parallel universes, Takasaki Koyomi lives with his mother after his parents’ recent divorce. Awkward and studious, he has trouble making friends at his new school until his classmate Takigawa Kazune seeks him out. She claims that she’s moved here from another world where she and Takasaki are in love… but could he be the one who’s actually from a parallel world instead?

To Every You I've Loved Before and To Me, The One Who Loved You are parallel universe novels by the same author and released on the same day. They can be read in either order to complement each other, and inspired a pair of anime films (also released on the same day).