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the first line you see 11 spins
and when you go to the other side you see 11 non-spine (2nd line)

or if its a trick question, as to books are "stacked vertically" there is one book in the stack and the inclusion of the information regarding spines is a "red herring".

Helena
I think you are right.I thought this and then
doubted myself.
You see 11 spines (with 12 non spines)
then on the other side you see 11 non spines.

so - 23

sorry I still count 12 on each side in your representation

N S N S N S N S N S N S N S N S N S N S N S N
your 1st view has 11 S(spines)

2nd view
S N S N S N S N S N S N S N S N S N S N S N S
has 11 N(non spines)

(if you started the other way round it would not work)

Gotcha! My brain is hurting now LOL

still can't see why a 'crossword' forum is hosting questions on logic etc -surely there are dedicated sites for those?

13 is correct
???? isn't !!

I think we can draw a line under this now, with the correct answer most likely to be 23 books maximum.(Depending on which side you look) as suggested by Frannie and Helena)

No it isn't 13 and yes I am right in that there are sites dedicated to puzzles like this on !! It's also a very good tricky question, more to it than meets the eye!!!!!