Take Back the Sky — Greg Bear

Marooned beneath the icy, waxy crust of Saturn's moon, Titan, Skyrine Michael Venn and his comrades face double danger from Earth and from the Antagonists, both intent on wiping out their growing awareness of what the helpful alien Gurus are really doing in our solar system.

Haunted by their dead and by the ancient archives of our Bug ancestors, the former combatants must now team up with their enemies, forget their indoctrination and their training, and journey far beyond Pluto to the fabled Planet X, the Antagonists' home world, a Sun-Planet in the comet-generating Kuiper belt. It's here that Master Sergeant Venn will finally understand his destiny and the destiny of every intelligent being in the solar system-including the enigmatic Gurus.