Sword Art Online 3: Fairy Dance (light novel) - Reki Kawahara

Kirito has escaped Sword Art Online, the forbidden VRMMORPG of death, and returned to reality. He heads off to find his game partner and lover, Asuna Yuuki. But Asuna never made it back from the nightmare. Lost and desperate, Kirito's only clue to her virtual whereabouts is a mysterious screenshot of Asuna as a fairy trapped in a giant birdcage. His conclusion: She must be trapped within the new, high-powered VRMMORPG, ALfheim Online. Now Kirito must plunge into the world of ALO, where players are fairies who fly gracefully through the air!