Skyward Flight (original) Brandon Sanderson

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 . . . and when FM's flight leader Jorgen finds a large group of Taynix - the space slugs then enable hyperdrives - they may have found their chance, if FM, Jorgen, the engineer Rig and the mysterious alien Alanik, can unlock the secrets in time . . . .