Then Woodward should release the tapes. If he doesn't, Trump haters will say "see, more proof" of something they already believe. Trump lovers will say, "see, more made up bullshit" by a press elite they already hate. The book will make Woodward some money, and won't do anything else.
Here's a good article that expands on this position:
Unless Bob Woodward Releases Tapes Proving His Book is Real, Fear Will Actually Help Trump