Orson Scott Card-- 42 books
Terry Pratchett-- 36 books
Isaac Asimov-- 31 books
C.S. Lewis-- 27 books
Neil Gaiman-- 27 books (including the Sandman series)
Stephen King-- 23 books
Diana Wynne Jones-- 19 books
John Grisham-- 18 books
Piers Anthony-- 16 books
Robert Heinlin-- 14 books
About 10 books each-- Rhet MacPherson, Jeff Smith, Plato, Gerald Morris, Kurt Vonnegut, Stephen Lawhead, Lloyd Alexander, Stephen R. Donaldson, Frank Herbert
However, there is one author whom I have read more than any other, and more frequently. I have read exactly 55 Dr. Seuss books (including 5 written as Theo LeSig and his adult book The Seven Lady Godivas), and some of these I have read more than a hundred times, such as The Cat in the Hat and Fox in Socks.