I can't find anywhere, not even the wiki, how bookshelves and levels correlate. All I get is that 15 is needed amount to make level 30 enchants. I assume that each bookshelf is two levels, but is that the case?
You could easily figure this out yourself by putting down one bookshelf at a time and checking in between.
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I must not fear. Fear is the mind-killer. I will face my fear. I will let it pass over me and through me, and when it has gone past I will turn the inner eye to see its path. Where the fear has gone there will be nothing. Only I will remain.