Hang Glider and Mud Mask

One lives high in the sky. The other lives deep underground. Follow Hang Glider and Mud Mask as they fly down and climb up to meet each other halfway--on the surface of the earth and in the center of the book. Hang Glider & Mud Mask is uniquely constructed with two front covers, two spines, and a Z-shaped binding that links the two sides of the story. Will you begin from the Hang Glider side, or flip the book around and start from the Mud Mask side? It's your choice. (And since one half's finale becomes the other half's front cover, it's hard to stop and put the book down.)