Nowhere Man wrote:wasn't he working on a sixth one when he died?
I believe he lost interest well before he wrote Mostly Harmless, which (I thought) sucked because of continuity errors* and the facts that Fenchurch disappeared at the beginning and everyone died at the end.
* e.g. Ford's maximally complete Guide entry on the Earth ("Plus the air is, for some reason, yellow.") which was eventually published in full, was mysteriously unpublished.