Goerge Langelaan
Some time ago I watched a Documentary on TV about the five Fly movies.
It seems that it's not a book that inspired the movies but a short story written by Goerge Langelaan in the 50s called The Fly . The story was published in Playboy and won the best fiction award for the magazine in 1957.
There might be a way to find the short story. It could be published elsewhere too. After all the movies were very successful! As soon as I know ow sth I'll post it.