The Amber Spyglass by Philip Pullman (2000) Yearling

Book three of His Dark Material wherein Will and Lyra fight Mrs. Coulter, Asriel and Metatron until they fall into an abyss and are destroyed.  They free the Authority from his crystal litter, but he is so old and powerless that the wind causes his form to dissolve back into dust.  But the two young lovers cannot exist in the same world.  They promise to go to the same bench in Oxford in each of their worlds.