Tegami Bachi, Vol. 7

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.
Lag has discovered that his long-lost hero Gauche Suede lost all of his heart, became tangled up in an anti-government organization called "Reverse," and is now known as "Marauder Noir." Noir’s trail leads Lag and Niche to the icy northern town Blue Notes Blues. It just so happens Niche was born here. Lag and Niche uncover her mysterious origins: a sacred underground lake...where her brethren still dwell.