Interpretations

[Mark Cecil]

I pretended not to look
as she carved her back The use of "carved" here instead of a more expected word like "curved" conjures up a greater image of the back being bent over and forces the reader to stop and think 
into a statue, and cracked it.

We were the only ones I would agree with what is already said before that this line falls a bit flat perhaps more focus on the fact their alone might create a bit more intimacy "we were alone in the library"
in the library, and I was reading
her vertebrae like a poem
I couldn’t understand.  I thought you could have a stronger ending linking closer to the name of the poem.  How about an additional line at the end: "I wonder if you could?" Or if you don't want to refer directly to the reader you could go "I wonder who could" It's just a suggestion.