"THE COLDEST WAR." By Ian Tregillis. Tor. $25.99.With "The Coldest War," Ian Tregillis continues an impressive fantasy/alternative-history trilogy he began with "Bitter Seeds." The first novel, set in Britain's darkest hours of World War II, pitted British warlocks, calling on dark otherworldly forces, against young Nazi Ubermenschen, whose X-men-like superpowers (invisibility, flamethrowing) are fueled by battery power fed directly into their brains.