All the Bright Places by Jennifer Niven

Theodore Finch is fascinated by death. Every day he thinks of ways he might kill himself, but every day he also searches for—and manages to find—something to keep him here, and alive, and awake.
Violet Markey lives for the future, counting the days until graduation, when she can escape her small Indiana town and her aching grief in the wake of her sister’s recent death.
 
When Finch and Violet meet on the ledge of the bell tower at school—six stories above the ground— it’s unclear who saves whom. Soon it’s only with Violet that Finch can be himself. And it’s only with Finch that Violet can forget to count away the days and start living them. But as Violet’s world grows, Finch’s begins to shrink. . . .