Godfall by Van Jensen

When a massive asteroid hurtles toward Earth, humanity braces for annihilation—but the end doesn’t come. In fact, it isn’t an asteroid but a three-mile-tall alien that drops down, seemingly dead, outside Little Springs, Nebraska. Dubbed “the giant,” its arrival transforms the red-state farm town into a top-secret government research site and major metropolitan area, flooded with soldiers, scientists, bureaucrats, spies, criminals, conspiracy theorists—and a murderer.