After 35 years in bookselling, Barbara Mead, former owner of Politics & Prose Bookstore in Washington, D.C., has retired. “On December 31 . . . I hung up my book bag,” she wrote in a letter on the Politics and Prose Web site. Since selling the store with Carla Cohen’s husband, David, a year and a half ago, she had been active in helping new owners Bradley Graham and Lissa Muscatine make the transition. “I’m tempering my sadness in leaving with my complete confidence that Brad and Lissa will sustain and grow P&P’s stature as an important literary and cultural center in today’s world,” she concluded.