The Keeper of Stars by Buck Turner

Who says you can’t rewrite the stars?

1962—Twelve years removed from the summer when she first gave her heart away, Dr. Elizabeth (Ellie) Spencer, Professor of Astronomy at Indiana University, receives a mysterious package that has her asking that very question.
Inside the unmarked box is a novel written by Ellie’s first love, Jack Bennett. It tells the story of a young man and woman who meet at the water’s edge and fall madly in love, only to have their romance cut short by forces beyond their control. Ellie quickly realizes this is no ordinary story; it’s their story. Captivated by Jack’s words, she immerses herself in the pages, reliving the adventures they shared that magical summer. To her astonishment, hidden among the tales of star-gazing and stolen kisses is a detail that has her questioning everything.
Seeking answers, Ellie turns to her sister for help, but before they can get to the bottom of the mystery, news reaches them of their aunt’s death, sending Ellie back to the water’s edge with the hopes of finding the truth and rewriting the stars.