BEGINNING THE WORLD AGAIN by Roberta Silman (Viking: $19.95; 416 pp.). Imagine the secret, cloistered world of scientists and their families in Los Alamos, New Mexico, working to create the first atomic bomb as World War II rages in Europe. This is the territory Roberta Stilman attempts to penetrate through the fictional eyes of Lily Fialka, one of the women on "the Hill." Although there are appearances by historical personalities including Robert Oppenheimer, Enrico Fermi, even Churchill and Roosevelt, Silman's true focus is on the wives of the scientists and their day-to-day lives in this tense, close-knit community. The writing has the intimacy of a diary and can be quite moving. But with so many accounts of the Manhattan Project written by the real physicists who participated--and who lived to regret their contributions--one doubts the relevance of these imagined characters, their ambivalent struggles, love affairs and reconciliations.