Blur, my second book is almost done. At this point the end is in sight, on the horizon. Then comes the editing and handing off to my beta readers and then after all that is done and I get all the feedback- nothing. I let it simmer. I don't go back to the book until I actually start to forget parts of it. Why do I do this? It gives me fresh eyes. It lets me approach the book as a reader not as the writer, which I have discovered has notoriously acute tunnel vision when it comes to my own writing.
I really suggest this as part of the writing process. The best way to let the book simmer is to get on the next book right away. Immerse yourself in your next project while the finished one is simmering. Then when you go back and rewrite and tweak and adjust and tighten the story, guess what is happening to the current project you are working on? It is simmering. So the cycle never ends.