It's shaping up into a hectic month for Robert Mason.

On Monday his first book, a Vietnam memoir titled "Chickenhawk," was published. By midday yesterday, the entire first printing of /10,000 copies was sold out. Next Tuesday, he will appear on the "Today" show.

And then he goes to prison.

The 41-year-old Floridian was convicted in 1982 on drug smuggling charges and sentenced to five years. Last month he exhausted his final opportunities for appeal and is awaiting assignment to a federal penitentiary--where he'll have ample time to pursue the goal he has had since boyhood.

"I'd always been writing," Mason said yesterday from his home in High Springs, Fla., "and it was always in the back of my mind that I wanted to be a writer." But he left college in 1965 to become an Army helicopter pilot, flying Bell "Huey" troop transports on search-and-destroy missions. Discharged in 1968, he wanted to write about his experiences, but "I didn't think it was achievable," he said. "I didn't have anything to say about Vietnam that I thought anybody would want to hear. And at that time, I was probably right."

After a string of odd jobs, he returned to the University of Florida and graduated with an art degree in 1971--despite, he says, heavy use of alcohol and Valium, frequent recourse to marijuana and an increasingly shaky marriage. He was treated by psychiatrists from the Veterans Administration--which had granted him a 50 percent disability "by reason of nervousness, now called Post-Traumatic Stress disorder"--and eventually started a mirror manufacturing business. He quit drinking and using tranquilizers and moved to New York City as an executive when his company was acquired by a larger firm.

After a few years, however, "I still hadn't learned to love mirrors." He took his savings and moved his family back to Florida where he built a cabin, determined to write about his Vietnam experiences. He had been encouraged by an old friend--Martin Cruz Smith, author of "Gorky Park" and, like Mason, part American Indian. Smith also introduced Mason to his agent, Knox Burger, who urged him to complete the project.

By 1976 Mason had finished four chapters, and Burger began to show the manuscript to publishers. After several had turned it down, "I was despondent about my future as a writer," Mason said. "In my mind at that time I had failed." Also worried about how to support his wife and son, he made "my big mistake." An acquaintance offered him a place on the crew of a sailboat carrying 1 1/2 tons of marijuana. The smugglers were arrested by South Carolina customs agents. Yet the time was oddly propitious. "After my arrest, I started writing another book, the novel I'm working on now." And shortly after his conviction, Burger called to say that The Viking Press wanted to buy the hardcover and paperback rights for an advance in the high four figures.

He finished "Chickenhawk" while appealing his conviction. The 339-page book is a keenly detailed evocation of the destructive might and desperate aerobatics of helicopter warfare (drawn from Mason's more than 1,000 combat missions), interspersed with anecdotes about the men in his company as war transforms their characters.

Although other publishing houses had told Burger that "Vietnam isn't salable," Viking's reaction was entirely opposite--and far in excess of the customary response to a first-time author. "It was one of those flurries of unreasonable enthusiasm," a Viking spokesman said yesterday, "and we expected an upsurge in the success of Vietnam books." Publishers Weekly called it "stunning"; Playboy said "outstanding." And Viking has ordered up another 6,000-copy hardcover printing.

"A lot of vets tell me it makes them feel like they're there," Mason said. "That's a great compliment."