Sci-fi parallel universes: New worlds created by as little as a cough

[Gertrude]

Charlie accidentally traveled to another universe that was very similar, maybe while he was sleeping. Through out the book he learned to travel and discovered that many of his alter lives had followed paths he had not chosen. But any number of things could cause a new parallel universe, even a cough.

He learned to travel among the lives and visit his other selves.

I read this book 20 years ago and mentioned it to a Physicist friend who was interested in it but have never found it again. I thought it was Dean Koontz but obviously I was wrong.