A sharp-witted young widow’s progress through post—Civil War Washington’s power-centers dominates this breezy and entertaining historical caper from the popular author of Henry and Clara (1994) and Dewey Defeats Truman (1997). Thirtyish Cynthia May, a math prodigy, exchanges her unfulfilling job as a secretary at the Department of the interior for a career as a human“computer” working in tandem with astronomers at the US Naval Observatory. Assigned to perform calculations related to the phenomenon of the transit of Venus, Cynthia finds a body of somewhat more interest in handsome young scientist Hugh Allison, who more than returns her interest. But Hugh is a visionary who, inspired by the comparatively new science of photography, dreams of transmitting visual images through space and time and Cynthia finds herself drawn into his romantic orbit. This promising situation is seasoned agreeably by the influential presences of Mary Costello, a lowborn fortune-teller determined to protect her own prophetic interests (“You scientific fellas are takin’ all the mystery out of the heavens”), and of the real-life New York Republican Senator Roscoe Conkling, a wily political animal who means to bend new President “Rutherfraud” B. Hayes to his will and add the fetching Mrs. May (a counterplotter as adapt as he) to his list of amorous conquests. Mallon recounts his characters— respective machinations with good-humored energy in an essentially well-paced narrative whose momentum is unfortunately undercut by heavy loads of background information attached to each successive character. Conversely, though, he composes unfailingly graceful sentences, makes transitions expertly, and communicates nicely both his characters’ and his own pure pleasure in the spectacle of a vigorous country newly at peace and pleased to kick up its political and sexual heels. Gore Vidal still does this kind of novel better than anybody, but Two Moons is a pretty fair incursion into his territory.