Tegami Bachi, Vol. 20

A New York Times best-selling sci-fi fantasy series with heart.
Amberground is locked in darkness. A man-made star casts only a dim light over the land. The pitch-black wilderness is infested with Gaichuu—colossal insects with metal exoskeletons. The Gaichuu make travel between the cities of Amberground extremely dangerous. But thankfully the Letter Bees, a brave corps of messengers, risk their lives in order to keep the hearts of Amberground connected.
What is the truth behind Amberground? As Lag and his friends descend to the center of their twilight world, they learn more than they ever dreamed possible. With civilization collapsing and the end of the world at hand, the time has come for Lag to lead the people of Amberground to hope and light at last.