CELEBRITIES ARE big business in the book industry, routinely securing boffo advances for their memoirs. Now there’s also a cottage industry of celeb publishers eager to cash in on the back end, too. Some imprints have been small, such as actor Viggo Mortensen’s Perceval Press, which focuses on the arts and criticism. Others are more mainstream. One certain to do well: Oprah Winfrey’s as-yet-unnamed Flatiron Books imprint, which will publish her memoir in 2017.