Horses and computers in latest Francis book; Twice Shy, by Dick Francis. New York: G. P. Putnam & Sons. 307 pp. $13.95.

Dick Francis' latest adventure has a different twist from his previous books. It is a rip-roaring story, but the characters aren't developed fully enough for the reader to get a real feeling for them. And the violence in ''Twice Shy'' seems more senseless and menacing than in earlier books.

Though Francis uses the world of horse racing as a background for everything from early-morning gallops to horse sales, there really isn't enough atmosphere for dedicated Francis fans. However, there is everything readers may or may not ever want to know about computers.

The first half is told by Jonathan Derry, a physics teacher interested in computers. His younger brother, 16-year-old William, is in school during this time.

Jonathan, through no fault of his own, has gotten mixed up with some murderous characters who are trying to steal a computer program designed to predict the outcome of races. Jonathan cleverly outwits the thieves and, thinking they are permanently out of the way, takes a teaching job in the United States.

Fourteen years later William's half of the story begins, and we learn he has been a jockey, but having grown too tall and heavy to ride, is now racing manager for a wealthy horse owner. William discovers that the thugs Jonathan bested have very long memories. The most vicious is out of prison, is still bent on owning the betting system, and doesn't care which Derry he goes after. Its up to William to outwit the crooks and save his own life.