A distaff Alienist, first-time author Lauren Belfer's hefty, fog-bound City of Light (The Dial Press; $24.95) exchanges Gotham for Buffalo but still features a Teddy Roosevelt cameo. Louisa Barrett is the beautiful blonde headmistress of the watery city's only girls' prep school, and an ideal amateur detective. Seduced by her access to Buffalo's devious industrialists (she's educating their daughters), Barrett hardly understands their schemes, never criticizes their attempts to make a quick buck on immigrant labor and the awesome power of nearby Niagara Falls. But Belfer can't decide whether she wants to be schoolmarm or gumshoe, delaying revelations with undigested bits of textbook history, short-shrifting Barrett's budding romances, and robbing the reader of a satisfying, big-bang action sequence. Like her heroine, Belfer's more at home in cushy drawing rooms than in power stations.