"It's one of Washington's longest running best sellers," Nancy Reagan said yesterday, accepting copy No. 1 of the 15th edition of "The White House: An Historic Guide," now in its 20th year as a principal moneymaker of the White House Historical Association.

"I hope the president won't be offended," said T. Sutton Jett, association president, during a presentation ceremony in the White House East Room, "but we feel copy No. 1 should be yours."

"I think I'll break the news to my husband gently," Mrs. Reagan replied, going along with the gag.

The occasion served a dual purpose, to mark the book--and the association's--20th year, and to present certificates of appreciation to nine individuals who made "exceptional contributions" to the original guide and its subsequent editions, which have sold a total of 3,638,206 copies since going on sale July 4, 1962.

WHHA board member Robert L. Breeden, vice president of publications, presented the certificates to three former White House curators, Lorraine Pearce (1961-62), William V. Elder III (1962-63), James R. Ketchum (1963-70); current curator Clement E. Conger; former White House chief usher J. B. West (1957-69); current chief usher Rex Scouten; current assistant chief usher Nelson Pierce; the editor of the 15th edition, the National Geographic Society's Donald J. Crump, and Nash Castro, with the National Park Service and first administrative officer of the historical association.

Among the nearly 100 guests were Smithsonian secretary S. Dillon Ripley; the National Gallery of Art director J. Carter Brown; National Geographic Society editor Wilbur E. Garrett, and Washington attorney Clark Clifford.