All that is holding the two books together is friction.  This is the same force that acts to stop you sliding a book across a table. When you interleave the two books there is a small force pulling the two books together created by the spine of the book.a table.  This compressive force will create a frictional force between every place the pages are interleaved, if there are 50 interleavings a small frictional force will be multiplied by 50 to create a huge force that you can't overcome with pure muscle.