A Washington artist and author was arrested by FBI agents at a Boston bookstore yesterday after he tried to sell rare letters written by Abraham Lincoln, Winston Churchill and others apparently stolen from the National Archives, federal law enforcement officials said.

Charles Merrill Mount, a 59-year-old art historian who had published three works on famed artists in the 1950s and 1960s but then fell on hard times, according to friends, was charged with interstate transportation of stolen property, authorities said.

The federal complaint alleges that Mount took from Washington to Boston nine letters written by the 19th century artist James Abbott McNeill Whistler, most noted for the portrait of his mother, that were discovered missing from the Library of Congress, and three Civil War era letters written or signed by Abraham Lincoln that were missing from the National Archives, according to James F. Ahearn, special FBI agent in charge of the Boston office.

On July 23, Mount had sold 25 historical letters, including the Whistler letters, to Goodspeed Bookstore on Beacon Street in Boston for $20,000. Later, the bookstore owners became suspicious of the origin of the rare letters and contacted the FBI, officials said. Agents called in a rare document specialist who identified the letters as ones he had sold to the Library of Congress in the 1970s, officials said.

Earlier this week, Mount again called the bookstore and offered to sell some vintage Civil War and World War I documents, officials said. When he arrived at the bookstore yesterday morning, FBI agents arrested him and seized an additional 75 documents, including hand-written letters from Abraham Lincoln and Winston Churchill.

Mount remained in the custody of federal marshals last night, pending a bond hearing today in U.S. District Court in Boston. He has not entered a plea.

More charges may be brought against Mount as federal authorities and art experts sift through the 100 historic documents, Ahearn said.

"We don't have a full understanding of what we have on our hands right now. We will have to put a dollar figure on it" eventually, Ahearn said. "We're asking any {rare document} dealers who are familiar with {Mount} to get in touch with us."

A letter dated Sept. 20, 1862, was penned by Lincoln to Maj. Gen. Henry Halleck, then chief of the armies of the United States, on white bond paper embossed in gray with "Executive Mansion Washington." Lincoln noted that five Virginia regiments "are reduced to skeleton" and that recruits from Ohio and other northern states were needed. "Could not the exchange be made? Yours Truly, A. Lincoln," the letter said.

In a Dec. 11, 1914, letter to an unidentified ambassador, Churchill, then first lord of the Admiralty in England, asked for news of his sister-in-law and other English ambulance nurses who had been detained and were returning to England.

In a postscript to the letter written on Admiralty Whitehall stationery, Churchill wrote: "I am sure you understand that I do not seek any special favor but only ordinary observances of war."

Officials described Whistler's letters as small works of art, signed as many were with an elaborate drawing of a butterfly, a hallmark Whistler frequently affixed to his canvases.

Yesterday, Mount telephoned an old friend in Gloucester, Mass., and told him the allegations are "absolutely not true," according to the friend, John Manship, an artist who has known Mount for 30 years.

According to Manship, Mount told him "the things {manuscripts} belonged to him for a number of years," that he had purchased them at auctions and that they would have been marked with a "Library of Congress seal" had they belonged to the institution.

Manship said his friend's arrest appeared to be the latest in a long tale of troubles for Mount, who has been engulfed in a series of court battles and a family feud that, according to authorities, ended with Mount's 1982 indictment on federal charges that he made telephoned death threats to his mother and another person. He served a short time in prison, Manship said. "That was the low point."

Mount also was involved in a highly publicized lawsuit in 1979, in which he alleged that he had not received payment for a portrait he painted of former Mississippi senator James O. Eastland.

According to Manship, Mount broke new ground with his 1957 biography of famed American artist John Singer Sargent, winning a measure of renown in the art world. In 1956, according to Who's Who in American Art, Mount held a prestigious Guggenheim fellowship.

Mount followed with a biography of Gilbert Stuart, known for his portraits of George Washington, and another on Claude Monet.

Manship said Mount became embroiled in controversy and made enemies in the art world during the 1960s and 1970s, but his situation seemed to improve in recent years, and Mount was working on another book.

A local art dealer described Mount as a man down on his luck who always appeared in need of money, a "kind of sad sack," whose paintings the dealer tried, but failed, to sell.

Mount's neighbors at the boarding house where he lived at 114 Fifth St. NE, which served as home to several interns and scholars, described him as a "lonely . . . frustrated scholar."

Along the way, according to one acquaintance, Mount traded his native New York accent for an English accent, and according to authorities, at some point he changed his name from Sherman Merrill Suchow to Mount.

For the last two years, Mount, often impeccably dressed and carrying a cane, had become a familiar sight at the James Madison Building of the Library of Congress, where he was registered as a special scholar with access to the first floor reading room where closely guarded original historic documents are kept, according to David Wigdor, acting chief of the library's manuscript division.

"The documents are used in a very controlled environment. They {readers} are not allowed to bring anything into the reading room but their imagination and a pencil," Wigdor said, adding that library officials were unaware that the Whistler letters were missing until the FBI called them this week.

Mount also had access to the National Archives because of his reputation as an art historian, federal officials said.

"Mount's a well-known portrait painter, a published author, the kind of person the Library of Congress deals with every day," said Kenneth W. Rendell, the historic documents dealer who identified the Whistler letters as belonging to the Library of Congress collection.

Rendell had identified as forgeries the Hitler Diaries in 1983, and more recently, forged documents sold to the Mormon Church.

"You can't set up a system that can beat" someone with a scholarly presence bent on stealing rare documents, Rendell said. "But what you can do is get the word out to dealers like Goodspeed Bookstore, so that when they sense something is wrong they call the FBI."