"Fools Die," the as-yet unpublished novel by best-selling author Mario Puzo of "Godfather" fame, brought a record $2,550,000 in the sale of its paperback rights yesterday to New American Library (NAL) of New York.

The sale, conducted by the book's hardcover publishers, G.P. Putnam's Sons, ended a 15-hour telephone auction, involving five paperback publishing firms, which at times had all the elements of a suspense novel.

The purchase by NAL, a subsidiary of the Times-Mirror Corp., eclipsed by $650,000 the previous record for a paperback sale. "The Thorn Birds." That was in March, when Harper and Row sold it for $1.9 million to Avon Books, also before the book's hardcover publication.

Included in the sale, though not a major part of it, according to a spokesman for Putnam's, were paperback reissue rights (known in the trade as reversion rights) to Puzo's earlier success, "The Godfather." It reportedly has sold 13 million copies.

Puzo's new book, a 598-page work that covers 40 years ending with the present, tells the story of a young writer who achieves huge successes at his craft and at the gaming tables in Las Vegas. The action moves between Las Vegas, New York and Washington, and includes a love affair with a Carole Lombard-type figure, according to William Targ, Puzo's editor at Putnam's.

There is one strong echo of "The Godfather" in the book, according to Targ in the character of an elderly owner of a large casino who know how to deal effectively with problems he encounters.

To be published by Putnam's Oct. 2, "Fools Die" has been chosen as a Book-of-the-Month Club selection. It is to be published in paperback in the fall of 1979. Puzo is to receive 45 per cent of the $2.55 Million, plus potential additional royalties depending on paperback sales figures.

The bidding on the book started at 9.30 a.m. Thursday in New York office of Irene Webb, subsidiary rights director for Putnam's. She began placing calls to the five active bidders: Dell, Bantam, Ballantine, Pocket Books and New American Library -- occasionally checking in with Faweet, the paperback house that published "The Godfather." Fawcett had made a $1 million "floor" bid, giving it an option to top the highest bid.

The 15-hour auction had executives hopping off planes to phone in bids, and by 5.30 p.m. Thursday Targ was able to phone Puzo at his Bay Shore, N.Y., home and asay, "We've top 'The Thornbird's' $19 million."

Puzo's reply: "Oh, that's nice."

At midnight, NAL had offered the highest price, with Ballantine the runner-up, a few hundred thousand dollars behind.

Elaine Koster, NAL editor-in-chief, said last night, "We're ecstatically happy about getting the book. We really love it, and think it's terrific, and we're proud to have Mario Puzo on our list.

The book reads like magic."

At Putnam's, the word was that the sale would make it "the hottest book around" for motion picture prospects.

Elsewhere in the trade, reaction ranged from incredulity to disgust, calling the $2.55 million sale price "wretched excess."

Puzo, who who was playing tennis throughout most of the auction, declined comment yesterday.

"He's a cool cat," Targ said."I wouldn't want to play poker with him. As a editor, I admire him. I don't know a single writer who's more surehanded. He listens well and then does a s he damn well pleases.Usually turns out right."

Having written the screenplays for the two "Godfather" movies, "Earthquake" and the t upcomming "Superman," and the treatment for Paramount's "Godfather III" that will star John Travolta as Al Pacino's son, Puzo has already decided that he will not do the screenplay for the film of "Fools Die." The movie rights sale is imminent.

"Would you like some advice for writers?" he recently said in an interview with Publishers Weekly, a trade journal. "If you go to a movie company, you go with a gun and mask. You have to stick 'em up ..."

And, as if foreshadowing all this hoopla, Puzo also told Publishers Weekly:

"I've never read a book of mine in print. After I'm through, I'm through.I like to get the checks. If don't get the checks, I howl."