it's not a trick Q
if u have 24 books thats 12 spine and 12 non(too many)
if u have 22 books - 11 spine and 11 non...but..
if u have 23 books u have 11 or 12 spine depending on what side u look at and the oppo side is the same , ie if you count the side with 11 spine the oppo side has 11 non!
Erm...I think that makes sense?