Terpsichore's post which inspired this one.
As will come as no surprise to anyone who knows my reading tastes at all, the author, of whose books I am most widely read, is PG Wodehouse. I don't exactly know how many of his books I've read, but it's got to be about 80-85. I own a few more than that (92); I haven't gotten around to reading the most recent acquisitions. I'll update with a picture later.
In a distant second is most likely Agatha Christie. I own 44 of her books, but I've probably read 50-55 when I include volumes checked out from the library. I mind very much less owning her books in cheap paperback editions.
Other notable mentions are Rex Stout (high 30's), Terry Pratchett (low-to-mid 30's), and Ngaio Marsh and Stan Sakai (31). Sakai is easiest to figure because I own all the books and I've read all the books. His are comic book collections, so if those don't count, we'll skip past him and Hergé (all 24 Tintin books) and light upon Patrick O'Brian (24 or 25).
I don't think I've read quite all of the 25 Albert Campion stories by Margery Allingham, but nearly. CS Lewis is at 19.Tolkien is muddled; is the author of The Silmarillion JRR or CR Tolkien? In any event, I'd put the number in the high teens. To get down out of double digits would take a significant list. Beverly Cleary, Isaac Asimov, Ray Bradbury, Gene Wolfe, Terry Brooks, John Keegan, Tom Clancy, Roald Dahl, Laura Ingalls Wilder, Lloyd Alexander... Some of those might not really count, being as most of their books are children's books, however. Anyone else want to chime in?