I've been working on my first novel for years. I'm sure it is in there somewhere, but the gestation period makes and elephant's look positively microscopic...
But I actually don't believe everyone has at least one book in them, even though my father used to say that often (he never wrote one, though he did write his memoirs). If you consider the huge number of non-readers in this world, I very much doubt many of them would have the inspiration or interest in producing one.
That said: