"Between the sheets" could be accurate, as pages used to be envelope-like, rather than double-sided.
In an Isaac Asimov short mystery, it's part of the solution. According to a man's aunt, his late uncle had said he'd left something of value in one of his unabridged books. Nobody could find anything. The aunt had 'elevated' the uncle's language, however, and said 'unabridged,' when the uncle has actually said, 'uncut' - referring to such pages.