Final Passage (The Prisoner and the Sun #3)
Despite years of searching, Iliff and Skye have neither seen nor felt the one Mountain rumored to climb beyond the clouds to the Sun. And Skye is fading. Now the challenge before Iliff is clear: complete his quest or lose Skye forever. But clues are few and cryptic, and all seem to point toward the last place he ever expected, or wanted, to venture.
Book III in The Prisoner and the Sun trilogy.