Spider Robinson has authored a new novel, Variable Star, with another author. His co-writer? Robert Heinlein. Apparently, archivist Bill Patterson discovered a detailed outline and Robinson went to work on it, scarcely believing the byline. This Wikipedia page has an overview of the plot.
I’m afraid Bester’s already taken. His unfinished novel, Psychoshop, was completed by Roger Zelazny.
It sucks, but I’m not sure whether this is due to the collaboration or the fact that all late Bester seems to suck.