Thanks to an indignant reporter who thought he'd gotten one book for the price of two, a complex plagiarism dispute has erupted in New York, pitting best-selling writer John D. MacDonald, creator of the Travis McGee series, and his publisher Fawcett against Avon Books and its author Dimitri Gat, who is accused of using large parts of a 1975 McGee mystery to write his latest novel, "Nevsky's Demon," published in May.

On Wednesday, Sandy Bodner, spokeswoman for the Ballantine/Del Rey/Fawcett division of Random House, said, "We have concluded that the Gat work infringes the copyright on 'The Dreadful Lemon Sky' by John D. MacDonald. We have informed Gat and his publisher of that fact. We are awaiting their reply."

Avon officials said yesterday that they are reviewing the claim as well as gathering information on the book's sales at Random House's request. "It is not our policy to discuss any matter which might involve potential litigation," Gat's editor, Judith Riven, said yesterday. "But we're very concerned and we're looking into it. We accepted the manuscript in good faith."

"I went through" Gat's novel, MacDonald said from his Sarasota, Fla., home, and "I find it difficult to believe that Mr. Gat was able to write his book without direct and frequent reference to mine. If that's actionable, so be it."

Gat, reached yesterday in Farmington, Conn., where he works as a procedures analyst for a manufacturing firm, did not deny that he drew from material from MacDonald's novel. "Not at all. In fact, I wrote him a letter earlier this week and told him that I had taken some parts of his book and worked them around to my way. I apologized."

All of which might have remained unknown were it not for the reading habits of Bob Sherman, a Washington staff associate for Jack Anderson, who broke the story last night on Metromedia television. Sherman, 35, commutes by train from his Montgomery County home and habitually passes the time with mysteries: "I buy 'em as I go through Union Station." One night last month he was reading "The Dreadful Lemon Sky" when he felt a jolt of de'ja vu. The passage he had just read--the account of a woman's death as told through a fictional newspaper article--seemed eerily familiar. "But I couldn't remember the name of the other book," Sherman said. Once home, he rifled his bookcase until he found "Nevsky's Demon," which he had read a few weeks before, and located the corresponding part. The words were different, the sense the same.

Intrigued, he bought more copies of both books, cut out the pages and pasted them up side by side. "There are differences in Gat's book," he says--particularly the Russian-American elements surrounding the hero, Yuri Nevsky, and the Pittsburgh location. (MacDonald's Travis McGee is a Floridian.) But he concluded that the plots were nearly identical, as was the cast of characters, with some differences in gender. Sherman felt "ripped off."

Both stories begin the same way: The hero is startled by the nocturnal arrival of a young woman who leaves a mysterious package in his care and warns that if she is not back within a month, it should go to a relative; within two weeks she is dead in an auto accident, and the hero sets out to discover if it was really an accident.

Many passages seem remarkably similar. Sometimes details vary. One of MacDonald's characters "came through the open door that led back to the warehouse portion. He had a clipboard in his hand." Gat's parallel figure "came up the stairs. He had been back in the warehouse section of the converted garage. He carried a half dozen lists . . . in a vinyl binder."

Sometimes the sexes of the characters differ. Here's MacDonald, page 47:

"He was okay. You know? A nice guy up to about a year ago. I've worked here since they opened. He drank, but like anybody else. Then he started drinking more and more. Now it makes him crazy. She's really a great person. It's really breaking her heart, you know."

"Booze sneaks up on people."

"It's made him crazy. The things he yells at her."

"I heard some of them."

This is Gat, page 46:

"Selina used to be a pretty nice person, believe it or not," she said. "Used to really care about what Tod was trying to do. Then about a year ago she found the money to hit the sauce and run off with one loser after another. Now she's driving Tod crazy, twisting the knife right in his heart."

"A drinking problem isn't a nice thing," Charity said.

"Think if you'd like to be yelled at the way she yells at him."

"I did hear some of it."

Sometimes the sense is the same, the phrasing different. On page 52 of MacDonald's book, a character is "so still I wondered if he was holding his breath. He licked his lips and swallowed and said, 'Two weeks ago?' "

"Does that mean anything?"

"Why should it mean anything?"

On page 50 of Gat's, it goes like this: His thin lips were set in a horizontal line. I wondered for several moments if he was going to speak. "Two weeks ago?" he said.

"Something special about two weeks?" I said.

"Should it be special?"

And sometimes, only the language changes. MacDonald's character says, "We're not open for business, friend"; Gat's says, "Monsieur-dame, we are pas ouvrier."

At first Sherman thought MacDonald was simply recycling his old books under a pseudonym and called the 66-year-old writer for an explanation. (Gat said Sherman had told him of the pen-name conjecture, and Gat mentioned it in his letter to MacDonald: "I said, 'Hey, I'm sorry I caused you that problem.' I certainly didn't want to have a guy like him accused of something like that.") After squashing that suspicion, MacDonald read Gat's book and later told Sherman that it seemed modeled on his.

Sherman took his materials to officials at Avon, talked to Gat and eventually to Fawcett. As a result, on Wednesday Random House's legal department sent an analysis citing 32 specific instances of alleged copyright infringement as "a partial listing of essential elements in the sequence of events and interplay of characters found in both books." Sherman said, "I feel really good. Everyone talks about the Anderson staff as being leak-catchers. But here's a story that's entirely self-generated."

Although Gat says he imitated MacDonald, he said yesterday that "I'm not sure what that means in terms of the more serious accusation" of copyright infringement. In general, copyright law does not protect ideas (such as plots or character types), but only the expression of them. After Sherman first called him, Gat said, "I consulted an attorney" and came away reassured that "it's very technical, this whole business. Scenes and even characters--to some extent you can't copyright them," and "certain things are kind of common domain." When informed of Fawcett's claim, he said, "Oh. That's awful news."

MacDonald, whose "Cinnamon Skin" tops The New York Times paperback best-seller list this Sunday, said that Gat's actions are "not the kind of thing that an old pro would screw around with. But this man is relatively new to the business, and may have thought it was customary procedure."

But Gat, who lives in Amherst, Mass., and is "45 or 46, I guess--the years go by so fast," said he is no novice.

Nor was he pressured by the deadline on "Nevsky's Demon," he said. "I'm fairly prolific, and I've been writing for a bunch of years. I can grind 'em out." Among other projects, he has co-authored a book on municipal grants, contributed to a volume called "Teachers in Transition," and written "The Shepherd Is My Lord," a science-fiction novel (Doubleday, 1971) and "Some Are Called Clowns," about a black baseball team in Indianapolis (Crowell, 1974).

Riven said she had never read "The Dreadful Lemon Sky," and took Gat's book at face value "as we do with all our authors--especially with one you've published before with success." "Nevsky's Demon" is the second volume in Gat's planned mystery series. The New York Times Book Review called the first, "Nevsky's Return," one of the "most notable crime novels of 1982" and said, "Mr. Gat is a real writer, and one eagerly awaits the further adventures of Yuri Nevsky."