Agent Nat Sobel caused a stir recently when he successfully auctioned a new thriller, set in Japan, to Japanese publishers before bringing it to market here. Sony was the winner there, and now Rain Fall by American lawyer-in-Tokyo Barry Eisler, has been preempted on this side of the Pacific for a "nice" six-figure sum for two books by Putnam senior editor David Highfill. He thus headed off an auction Sobel had planned to close this week. The book stars a Japanese-American assassin, John Rain, who becomes the target of government agents after one of his jobs is completed. "The ominous Tokyo noir atmosphere is the icing on the cake," Highfill said. He bought North American only, and Sobel will be offering the book at Frankfurt.