In those twenty years, the magazine has covered books fairly well, but has showed authors on the cover rarely. (They have run dozens if not hundreds of cover stories about movies based on books, but those show actors, not authors.)
I can recall only three cover stories with the author him- or herself in the spotlight (from 1994, 2002, and 2007, respectively):
The current issue is, I believe, the second time the cover has depicted not an author but a book itself:
I realize this is a milestone only in my narrow little pop culture realm, but it's still a nice thumb's-up for the publishing industry.
In this 4/6/12 issue, editor Jess Cagle writes "Why aren't there more book-themed covers on EW? The answer is obvious: If you've got an overall audience of nearly 18 million (including our magazine and website), movies and television are the fastest way to the greatest number of hearts."
(Turns out this Fifty Shades of Grey cover was for subscribers only; the cover of the newsstand edition of the same issue featured The Hunger Games...film.)