Skyward Flight by Brandon Sanderson & Janci Patterson

In Sunreach, after a planet-destroying Delver suddenly appears in the sky of Detritus and vanishes just as suddenly, FM knows that the last free human society got lucky. Her Skyward Flight companion Spensa figured out how to draw this Delver away, but it won't be so easy next time. Humanity has to be prepared.

In ReDawn, Alanik has recovered from the shock of answering a distress call and finding a planet of humans making a stand against the Superiority, only to be dismayed to discover they're considering a peace overture from their enemy.

Worse, when she returns to her home planet of ReDawn, she find her own people falling into exactly the same trap. With her mentor captured, she turns to her new friends to help: can Alanik, FM, Jorgen and Rig coax an ancient technology into life in time to save both their planets from disaster?

And in Evershore, the government of Detritus is still in disarray following Superiority treachery, and no word has come from Spensa, on her mission deep in the Nowhere, leaving Alanik, Jorgen, FM and Rig to pick up the pieces.

So when the Kitsen send word from the planet Evershore, saying they have some humans and wish to return them, they have to decie if the strange message can be trusted.