Sam's Letters to Jennifer by James Patterson
Jennifer is still reeling from the death of her husband Danny when she gets the call that her beloved grandmother (and best friend), Sam, is in a coma. Jennifer rushes to Sam's side and the lake house where she spent much of her childhood. She finds a stack of letters from Sam to her that tell Sam's secret love story. Reading the letters not only brings Jennifer closer to Sam but also opens her heart to allow herself to find love again.