you have a few options.

open the Joint editor in Poser, change the Orientation angle slightly while the book is open. Depending on what dial is opening the book you have to use either X Y or Z. make sure the axis ( Green ) is on top where it is not torn apart else it will cause that both sides split ( Move the axis to the top corner ) .

second option as it seems that you use the bend off feature for this articulation would be to merge the edges L + R of your book can be done in Blender or other 3d program in this case you would rig it bendable, a little more work.