When a letter from John W. Hinckley Jr. arrived at the New York Post in 1982 -- "Don't turn me into a monster," Hinckley complained -- reporter George Carpozi had his literary agent write back, urging Hinckley to cooperate with a book project.

Now President Reagan's would-be assassin, according to Carpozi, has agreed to help in exchange for 25 percent of the profits. And Carpozi, now deputy news editor of The Star, a supermarket magazine owned by Rupert Murdoch, is hard at work on a tome to be titled "The Day I Shot the President: The John Hinckley Jr. Story."

"Someone has to do it," Carpozi said yesterday, adding that his agent is currently shopping around for a publisher. "I feel I have to perform this service. His story has to be told. You can say I don't feel exactly happy that he would make any money out of this. It wasn't my idea. It was his. I knew that without consenting to that kind of deal, he would just tell me to go fly."

The recently passed Comprehensive Crime Control Act of 1984 forbids convicted assassins and other criminals from making money off the sale of their stories, but Hinckley may be entitled to collect, says assistant U.S. attorney Charles Roistacher. "One problem is that he wasn't convicted, he was found not guilty by reason of insanity," Roistacher says.

The White House had no comment yesterday.

Hinckley, who shot the president and three others on March 30, 1981, has been committed indefinitely to St. Elizabeths Hospital.

Carpozi, 65, describes himself as the author of about 70 books, including "Red Spies in Washington" and "Frank Sinatra: Is This Man Mafia?" He says Hinckley has been corresponding with him regularly since the first letter 2 1/2 years ago, sending narratives from the mental hospital.

In one such narrative, excerpted in yesterday's New York Post, Hinckley wrote: "I would either shoot President Reagan and be killed in the attempt, or else be taken into custody and have my life changed forever. The only other alternative was to take the bus up to New Haven and shoot myself in front of Jodie [Foster]."

Carpozi says Hinckley, a compulsive writer of letters to reporters, first mailed one to the New York Post to complain about an article on him: "I liked the stuff about Jodie but the rest was rather critical."

"I think that it was one of the most tragic occurrences that was ever seen," Carpozi says of the shootings. "But I've had to watch three men in Sing Sing go to the electric chair at once. I didn't enjoy watching it, and the stench from that execution stayed with me for six months. But it was a job I had to do."