This one is quite ominously named, but don't worry, it's for good reason. The closed book is when one or both members of a couple sleep on their stomachs. To be clear, this only refers to people who never previously slept on their stomach when they were single but happen to do it when they're sharing a bed with their partner. Even people who do this have to admit it's a little weird to change your sleeping style like that.
The reason there is some cause for concern here is widely agreed upon by all kinds of experts - sleep experts, relationship experts, and even chiropractors have expressed worry about the belly sleeping couple. In case you haven't guessed yet, sleeping on your stomach can indicate that you're anxious, insecure, and closed off in your relationship.