Concealed behind books (6)
I believe the answer is:
latent
'concealed' is the definition.
(I've seen this in another clue)
'behind books' is the wordplay.
'behind' becomes 'late' (as in behind schedule).
'books' becomes 'NT' (New Testament).
'late'+'nt'='LATENT'
(Other definitions for latent that I've seen before include "Concealed - not yet developed" , "Potential but not active" , "As yet undeveloped" , "Existing but not yet developed, say ability" , "Hidden (talent)" .)