Yes. That space where the bent book opens to reveal that "hole". You're exactly right. There's not a good word for that hole that appears when an old book is opened, but if the book is big enough, quite a lot can fit in there.
On the earlier comment: Agree fully that I intended "Aren't you the lucky one then..." to be meant as "Aren't you fortunate not to have this problem ([that we are discussing]?"