"Spoken from the Heart" by Laura Bush /Betty Ford shows off, "The Times of My Life.” / “It Takes A Village,” by Hillary Rodham Clinton. (Simon & Schuster/Dave Pickoff (AP)/Simon & Schuster)

Michelle Obama’s appearance in the White House garden Wednesday wasn’t just a photo-op: It was also a clever way to promote her new book.

Within hours of the spring planting, Crown Publishing Group announced that the first lady will pen her first book, due out next year, about the South Lawn garden and (no surprise) her personal mission to introduce kids and their parents to fresh, healthy food.

Obama joins plenty of previous first ladies who added “author” to their resume while living in the White House.

“These are very smart people who understand their issues, know how to speak and write about those issues, and are passionate about getting the word out,” said Bob Barnett, D.C.’s go-to-lawyer who brokered this (and every other big political book deal.) Obama is not taking an advance and will give all her proceeds to charity.

There’s a long tradition of first ladies publishing their books while still living at 1600 Pennsylvania Avenue. These aren’t the juicy, score-settling memoirs (those typically come after their husbands leave office and almost always become bestsellers) but charming, feel-good tomes designed to raise awareness and spirits.

But still valuable, said historian Carl Anthony: “They provide an insight into the thoughts and personality, how opinions of these women have been shaped.”

Rose Cleveland, the president’s sister who served as first lady, was the first to take advantage of her position, publishing both a book on George Eliot’s poetry in 1885 and another on the moral, intellectual and social culture on American life. Eleanor Roosevelt wrote several books during her White House years including 1937’s “This Is My Story,” the first of a three-volume autobiography. It was a huge hit, thanks to Roosevelt’s radio, newspaper and magazine presence. “She was the original cross-marketer,”said Anthony (who blogs at carlanthonyonline.com).

Barbara Bush shows off “Millie’s Book.” (Mark Lenhinan /AP)

And now Obama’s garden book, which Anthony said could be a “very interesting piece.” But will he buy it? “I just might. I’m kind of curious about it.”