The League of Seven — Alan M. Gratz

In an alternate 1875 America electricity is forbidden, Native Americans and Yankees are united, and eldritch evil lurks in the shadows. Young Archie Dent knows there really are monsters in the world. His parents are members of the Septemberist Society, whose job it is to protect humanity from hideous giants called the Mangleborn. Trapped in underground prisons for a thousand years, the giant monsters have been all but forgotten--but now they are rising again as the steam-driven America of 1875 rediscovers electricity, the lifeblood of the Mangleborn. When his parents and the rest of the Septemberists are brainwashed by one of the evil creatures, Archie must assemble a team of seven young heroes to save the world.