Nick of Time. The Gunn Files, #4

This timeline is set to expireWhen Gunn investigates a blood-soaked crime scene without a body, he stumbles into a time-torn battle for Earth's future. Now, he's caught between warring factions: a charismatic alien revolutionary promising Earth's freedom, and the galactic Federation's inscrutable iron grip. The key to it all is a temporal device of alien manufacture. It can flood streets with water from the past-or reshape humanity's future-and it's fallen into enemy hands.
With the help of a rogue Peacekeeper, an underground resistance, and his quick wit, Gunn races to recover the stolen device before time runs out. Because someone's about to rewrite Earth's history, and humankind might not survive the edit. Nick of Time is the explosive fourth book in The Gunn Files, a fast-moving science fiction joyride that combines noir detective fiction with humorous sci-fi in the tradition of John Scalzi and Douglas Adams.