Paramount Pictures has bought Caleb Carr’s novel “The Alienist” for producer Scott Rudin, who just extended his studio production pact, putting off his announced plan to move to TriStar for at least another year.

While neither Rudin nor Paramount exex would divulge financial details of the deal — hammered out by International Cre-ative Management’s Irene Webb in conjunction with ICM’s Suzanne Gluck — sources confirmed that Par shelled out close to $ 750,000 for the book. The buy is significant in that it was an outright purchase, as opposed to an option.

Although there was considerable interest when ICM sent the book out last week to a number of producers, Rudin immediately made a preemptive bid.

Random House will publish “The Alienist” next April. Set in turn-of-the-century New York, the story features historical and fictional characters using the early principles of the emerging discipline of psychology to track down a gruesome serial murderer.

Sources said that a screenwriter will be hired to adapt the book.

Author Carr’s “The Devil’s Soldier,” also published by Random House, was optioned by Richard Gere and producer Gene Kirkwood. The project is in development. Carr has written other historical novels and TV scripts.

Rudin was to move his production company to TriStar Pictures next month when his Par contract expired, but will now stay on at Paramount to finish the slate of films he has in development and production on the Hollywood lot.

In addition to “The Firm,” which opens Wednesday, Rudin has a number of projects at the studio, including “Addams Family Values,” the sequel to “The Addams Family,” which he also produced and which will be released Nov. 19, and “Searching For Bobby Fischer,” which opens in August. Rudin also has projects in development at Paramount, including “The Cheese Stands Alone” and “Nobody’s Fool ,” with Robert Benton is attached to direct.