Ever by Gail Carson Levine

Falling in love is never easy, but falling in love with an immortal god while your days on earth are numbered is almost more than a young girl can bear. Newbery Honor author Gail Carson Levine has created a stunning new world of flawed gods, unbreakable vows, and ancient omens in Ever.

Kezi knows her days are numbered. But accepting death is hard, especially when her romance with Olus is so new—Olus, the god of wind and loneliness. 

Love brings Kezi the will to fight her fate. Love gives Olus the strength to confront his fears. Together—and apart—they encounter spiders with webs of iron, the cruel lord of the land of the dead, and the tests of the Akkan gods. 

If they succeed, they will be together. If they fail, Olus will have to endure the ultimate loss—and Kezi will have to make the supreme sacrifice.