WASHINGTON – The letter is marked “personal and private” and is addressed to President Franklin D. Roosevelt’s secretary, Grace Tully, who was with the ailing chief executive in Warm Springs, Ga., that Thursday in 1945.

The writer was Lucy Mercer Rutherfurd, who decades before had been FDR’s mistress and who now was making arrangements for what would be their last fateful meeting at the president’s rural retreat.

Elegantly handwritten, the letter never mentions Roosevelt by name – her love letters to him had been their undoing a quarter-century earlier. He is just “the subject,” or “the ‘B,’ ” for boss. But the arrangements worked out, and a week later the former lovers were together again on the day he died.

The letter is part of a newly acquired trove of 5,000 pages of Roosevelt documents that the National Archives said Wednesday should be a feast for historians of the president who led the nation through the Depression and most of World War II. A YouTube video by the National Archives details some of the letters and the story behind the acquisition of the trove.

The 12 boxes of material had been in private hands and never available to scholars or the general public, said Bob Clark, supervisory archivist at the National Archives’ Franklin D. Roosevelt Presidential Library and Museum in Hyde Park, N.Y.

They were saved from the auction block by special federal legislation this year that cleared the way for their donation to the National Archives, according to Sen. Charles Schumer, D-N.Y., and Rep. Louise Slaughter, D-N.Y., who co-sponsored the measure.

Archivists hope to have the collection publicly available by November and online by January.

“Wow,” said historian Doris Kearns Goodwin, author of “No Ordinary Time,” a chronicle of the Roosevelts during the war. “This stuff sounds like it’s going to be very exciting. You very rarely get a whole new trove of material. ... It’s pretty great.”

The documents come from FDR’s intimate professional inner circle: his two chief secretaries, Marguerite “Missy” LeHand and her successor, Grace Tully. Both women were virtually part of his family, Goodwin said.

And judging by the sample of nine documents unveiled at the National Archives on Wednesday, the material has a behind-the-scenes feel as if fresh from the president’s desk.

Clark said they include the private papers of Tully and LeHand as well as Roosevelt’s papers, which Tully took when she left the White House after FDR suffered a stroke in Warm Springs and died April 12, 1945.