Anonymous who wrote “My Secret Life.”
Kesey also wrote Sailor Song 28 years after Sometimes A Great Notion
But yeah, I thought of him immediately as SAGN is one of the best books I’ve ever read.
Nah, there’s lotsa stuff by Anonymous. 
Including this political best-seller: Amazon.com
Lately… becoming David Baldacci. His last few books have really sucked. The Amos Decker books are practically unreadable.
Baldacci has published more than 30 novels and he still puts out 1 or 2 every year. If he’s not prolific, I don’t know who is.
Anne Frank
He’s written three books in 10 years. Not super prolific output, but not terrible.
He does seem to have a bit of a Martin problem, in that he seems to like being a famous fantasy author a lot more fun then sitting down and writing another fantasy book. But he still has some output, certainly not in the same league as other authors mentioned here, who just turned out one best-seller