Reader's Digest has reached an agreement to sell its Chappaqua, N.Y., headquarters to two real estate groups in a transaction in which RD will receive $59 million in cash. As part of the deal, RD has signed a 20-year lease to continue to house its corporate staff on the property. It will occupy about one-third of the 700,000 square feet of space in the headquarters building. RD said it will take about one year to "restack" the portions of the facility it will use. RD had put its headquarters up for sale earlier this year after several years of downsizing and diversification made the 114-acre campus too large for its needs.