It's 1947, and Branch Rickey is about to change the all-white face of baseball forever: He is bringing in Jackie Robinson to play first base for the Brooklyn Dodgers. During spring training in Cuba, Robinson receives death threats and is followed, while Daily News sportswriter Joe Tinker wonders whether there's a connection between the threats and the murder of an ex-middle-weight club fighter, who may have been killed for not fulfilling his role as Jackie's assassin. The trail ties in with another death, that of hero cop Harry Wilson, who was shot in a black prostitute's Greenwich Village apartment. Harry's brother, rabidly anti- ''coloreds,'' has just returned from Cuba, and while Robinson prepares for opening day, the threats escalate. Tinker uncovers Harry's unconventional (for the 1940's) love-life and how it influenced his hero-worshipping younger brother--leading to a sniper in Ebbets Field who's ready to gun down Robinson unless Tinker can stop him. Earnest, but one could blow Cuban-cigar rings through the holes in the plot (a Day of the Jackal spinoff), and the '40's ambiance is negligible. Honig (Marching Home, 1980, etc.) is strong on baseball lore, weak on characterization and poetics.