By MICHAEL CAINES
The novelist Isabel Colegate is best known for The Shooting Party, published in 1980, in part because of the film adaptation that quickly followed. Her first novel, The Blackmailer, has a more interesting claim to fame, a story from the small world of mid-century publishing that I believe she's written about before, but which I only discovered from the reissue published last year with a new foreword. . . .