This is a very strange sentence, and it is nearly impossible to know the meaning without seeing more context. However, I was able to find the context, a short piece of writing on this page of this book: https://books.google.com/books?id=_-PzdXhnhLIC&pg=PA158 In this piece, the author is talking about the long and difficult process a writer may take from a small and vague inspiration all the way to a completed book. So the author is using 'yeast' in a very artistic and unusual way - I have never seen the word used like this before - to mean 'inspiration', for writing, in this case. This was only clear to me from reading the entire paragraph, which is about authors who write a book then still can't get the details of their story out of their head, like there is something about their own writing they still haven't discovered. The literal meaning of 'yeast' is a substance used that is necessary to make bread. So here, 'yeast' is a metaphor: in the same way that yeast is used at the beginning to eventually create bread, an inspiration is necessary at the beginning to eventually write a book. The way that the 'yeast' (or inspiration) outlives the book, is when the author cannot move on from the ideas in the book, and they become an obsession. Again, this is not obvious from that one sentence alone. Reading the entire paragraph was necessary for understanding this one sentence.