THIS YEAR is the 50th anniversary of Irma Rombauer's The Joy of Cooking, nine million copies of which are in print, but Bobbs-Merrill, the publisher, has interrupted its celebrations to announce that there's a new author on its list: Nancy Reagan.

The First Lady has signed a contract for a book to be called To Love a Child, which will feature the personal experiences of a dozen participants in Foster Grandparents, a program under the ACTION umbrella. She has been a friend of Foster Grandparents since she and Ronnie first reigned in California, and though it began as a federal program, Mrs. Reagan, true to her husband's ideology, helped get the first state funding for its efforts.

Although the news stories reporting the contract signing say that all the royalties from To Love a Child will be going to the Foster Grandparents program, the truth appears to be that it's going to be most and not "all," according to Bobbs-Merrill and Mrs. Reagan's press office.

No writer has been selected for the book yet, and Jack Kenyon, the director of Foster Grandparents, is waiting to hear from the White House about what kind of cooperation it wants from his people. Meanwhile, an increase has been requested by Congress for the Foster Grandparents' budget, and Kenyon's not complaining. "It's small, but any kind of increase at all is miraculous in these times."

Though Kenyon knows who his fairy godmother is, he knew nothing about the book idea until he heard it from a member of Mrs. Reagan's staff when he was out in California to tape a Mike Douglas show with Mrs. Reagan. "That's okay," he's quick to point out, "it's her book." Grace Shaw, publisher of Bobbs-Merrill and the First Lady's editor, agrees. "Mrs. Reagan will be very personally involved," she stresses. More than that she will not say, except that To Love a Child should go over big with the "Reader's Digest audience."

The agent for the book is Bill Adler, who also handled Nancy's Nancy. In case Adler's name doesn't sound familiar, he's the guy responsible for The Kennedy Wit, Kids' Letters to President Carter and The Wit and Wisdom of Bishop Fulton J. Sheen.

Adler's The Reagan Wit has already hit the shelves, but somehow I don't think we're going to get Welfare Mothers' Letters to President Reagan as a followup.