Sen. Sam Nunn (D-Ga.) last night sent a strongly-worded letter to President Reagan questioning the propriety of Richard Perle's efforts to sell a book proposal while serving as assistant secretary of defense, as bidding for the proposed novel continued for a second day.

"One cannot have it both ways," wrote Nunn, ranking Democrat on the Senate Armed Services Committee. "One cannot claim that the published account is merely fiction while at the same time marketing the book as so authoritative and proximate to the locus of power in Washington as to be worth in excess of $300,000 . . .

"Perle must chose between remaining one of the principal architects of U.S. security policy or undertaking to become a best-selling novelist."

A government ethics official indicated yesterday that Perle is subject to rules that prohibit a federal official in his position from using public office for private gain, or from earning more than 15 percent of his salary in outside income. Perle earns about $70,000 a year.

Perle responded that he did not intend to write the novel or to accept any money for it while still serving in the Pentagon. He said he had no plans to leave his job soon.

The Washington Post reported yesterday that Perle's agent was circulating among publishers a five-page outline for a novel, tentatively titled "Memoranda," that seemed to offer a fictionalized account of Perle's longtime conflict, in Reagan's first term, with former assistant secretary of state Richard Burt. The bidding reached $300,000 on the first day of the auction, and was reported by publishing sources to have surpassed that sum on the second day, though it could not be learned whether the auction was concluded.

In an interview yesterday, Perle denied there was any impropriety in his selling the rights to his novel now. "My thoughts about novels will not interfere with my conduct of business," he said. "Publishers were made aware in writing that I would not receive any money until after I left government."

About Nunn's letter, Perle said, "I wish he had picked up the phone. Nunn is assuming that I am going to say improper things . . . I am not about to write a book that will improperly disclose sensitive, substantive issues." He also said, "I seem to remember that two of his colleagues on the Armed Services Committee wrote a novel." He was referring to "The Double Man" by Sens. William S. Cohen (R-Maine) and Gary Hart (D-Colo.). Senators, however, are governed by a different set of ethics rules than are federal employes like Perle.

At issue are an executive order prohibiting federal employes from taking actions that "might result in, or create the appearance of using public office for private gain," and a regulation stating that certain officials cannot earn more than the equivalent of 15 percent of their salaries from outside sources. A memorandum about book earnings issued in 1982 by the Office of Government Ethics says that "an arrangement whereby such advances would be staggered or deferred would not avoid" the 15 percent limitation.

Ethics office director David H. Martin declined to comment without knowing further details of Perle's plans.

"A great deal of [the novel] will be based on real events in the world," Perle said, "but no one individual serves as model for a character in the book."

As for his ability to portray official Washington in fiction, he said, "I think I am not untalented as a writer."