Historian Daniel J. Boorstin's previously announced plans to devote his retirement years to writing apparently will be postponed, or fulfilled in the off-hours.

The former librarian of Congress, who relinquished his post last week after 12 years, will join the Doubleday publishing company as its editor-at-large, a position created especially for him. He will be based in Washington.

According to a statement by Nancy Evans, president and publisher of Doubleday, Boorstin will be acquiring books in the fields of history, reference and biography "and wherever else his imagination and talent lead him and us." He will assume his position Oct. 1.

The appointment is a fresh coup for Doubleday. Until its acquisition late last year by Bertelsmann, a West German publishing conglomerate, the 90-year-old publishing company was perceived to be adrift. Its editors were leaving and taking their authors with them, and Doubleday was losing the competition for best sellers, often to Random House and Simon and Schuster.

The arrival of Evans last February -- from the Book-of-the-Month Club, where she had been editor in chief -- was taken as a sign of the company's serious commitment to trade publishing. Boorstin's appointment suggests that the intentions have some substance. The post of librarian of Congress has been compared to a U.S. "minister of culture"; Boorstin's books have won the Bancroft, Francis Parkman and Pulitzer prizes.

The Doubleday statement also said the new editor-at-large, who will turn 73 the day he takes his new post, would be recommending republication of titles on the Doubleday backlist, books "that remind us of our political and literary heritage and stir us to keep it alive and thriving," in Evans' words.

Whatever his new obligations, the statement said, Boorstin plans to continue working on "The Creators," the companion volume to his 1983 book, "The Discoverers." Random House will publish the second book just as it did the first, the statement noted. Boorstin was en route to the Netherlands yesterday and unavailable for comment.