Not to mention, off the top of my head...
Sir Arthur Conan Doyle (Holmes: four novels, 56 short stories)
Ian Fleming (Bond: 12 and two)
Lilian Jackson Braun (The Cat Who... murder mysteries, 29)
In current books, I am a fan of David Weber, and his Honor Harrington series has passed 14 books if you count the anthologies(although some feel it has stretched too far, and I can understand that. )
You could as easily argue that it takes a particular talent to be able to continue a series so long and keep it fresh enough to remain enjoyable.
Certainly does, though I never got into Weber's writing.
I couldn't quantify what it is about Weber that appeals to me; I can actually point to flaws or problems with things, but I find it much harder to specify what I enjoy about it. I find it extremely easy to read, which I consider the best compliment one can give a work of fiction. I get into the story, I don't want to put the books down. I overlook or don't notice the flaws because of how easily I can slip into the world he creates.
It's the same with movies I really enjoy. I can more easily find the flaws than the things that work particularly well, and I think that's the point. A good work of fiction usually isn't about specifics so much as an overall feel for me.