I never got or kept a record of the author or title of a book that came out about 25 years ago, but my recollection is the author had turned up the official inquiry records concerning Marlowe's death, and put to torch all speculation that he didn't really die.

I was wrong about the earliest date of a Shakespeare play - c 1591 for Henry VI.

As to had a hand in someone else's plays, there will never be an end to speculations of that sort.