It gets exhausting to work on a book. There is this myth that authors must hide away in a garret and work slavishly away at their manuscript before offering it up to the world. (I have a lot of issues with this myth, but I'm only focusing on one right now.) It is important to take a step back away from the Great Work and rest. If you force yourself to be 'on' all the time, you face the risk of burn out.
We're all familiar with the effects of burn out. We feel hopeless, over pressured, tired, as well as mentally and spiritually drained when burn out hits. It is important to take some time to take care of yourself when you are a creative person. As tempting as it may be to focus strictly upon the ecstasy of creating, we need to remember that all things must be taken in moderation.
Sitting at a computer typing all day with out any breaks will make you have headaches and eventually develop carpal tunnel syndrome. The same is true for writing out things longhand or pretty much anything else that involves repetitive stress of that sort. Sometimes, the brief break of looking away from the computer at something across the room or outside is just the thing you need to rest your eyes. Sometimes taking fifteen minutes to rest your hands and wrists helps make them feel more limber and able to resume work.
But there are times where you need more time to rest than those fifteen minute breaks. Sometimes, you have to put that project in a drawer and forget about it for a little bit. Taking a complete break from the project allows you to rest your mind for a little while. You can work on other projects (ideally completely different from what you were to begin with) or you can do something completely separate from what you were working on (like baking cookies instead of writing). This divorce from your project untangles your thoughts and allows you to rest.
Rest and recuperation are important to anyone who regularly exerts themselves. It is just as vital for an artist or an author as it is for someone who lifts weights. And it is for similar reasons. Taking time to rest helps you regather your strength. It allows allows your muscles to recover from the strain that you were putting them through. In the case of an artist or an author (or anyone else who does something that is a much more low impact activity than lifting weights) what you are allowing to recover from the exertion is your mind. (And what ever you are using for your work.)
It is important to remember that the activities that help us to recover from extended mental effort are going to be different for every person. Some one may find that sitting and reading the cheeziest books they can get their hands on is a perfectly restful activity that leaves them feeling refreshed and ready to get back to their creative work. Another person may find coloring does so (and thank goodness for the trend making it 'ok' for adults to do this relaxing activity). Still another person might find that they feel best after some quiet meditation or a walk in the woods. And this is ok, because every person has a different way they look at the world and differences in how they process it (which leads to difference in their creative output).
Taking a rest and doing things that help us feel more creative is important for our work. It gives us new eyes and new ideas for how to approach it. It helps us to approach our work with greater clarity. And, when we've been upset with how it was progression, greater calm. So, take a moment, get your beverage of choice, kick your feet up, and relax for a little bit. Your work will be better for it and you'll be healthier. Everybody wins!