The Summer I Turned Pretty by Jenny Han .

The Summer I Turned Pretty by Jenny Han is a bestselling young adult coming-of-age romance novel and the first book in the popular Summer trilogy.

Isabel "Belly" Conklin measures her life in summers. For as long as she can remember, every year has been defined by the months spent at Cousins Beach in a shared beach house with her mother, her older brother Steven, her mom's lifelong best friend Susannah, and Susannah's two sons: Conrad and Jeremiah.

For Belly, the beach house is magical, and the two Fisher boys have always been her childhood best friends, secret crushes, and constant companions. But the summer she turns 16, everything shifts. Suddenly, the dynamics change, old childhood lines blur, and Belly finds herself caught in a complicated, emotional love triangle between the two brothers as they all face a summer of growing up, first love, and unexpected heartbreak.