On reflection, books are a source of inspiration (5)
I believe the answer is:
erato
'a source of inspiration' is the definition.
(I've seen this before)
'on reflection books are' is the wordplay.
'on reflection' says the letters should be written in reverse.
'books' becomes 'OT' (abbreviation for Old Testament).
'ot'+'are'='otare'
'otare' in reverse letter order is 'ERATO'.
(Other definitions for erato that I've seen before include "Greek muse - orate (anag)" , "poetical inspiration" , "daughter of Zeus" , "Poet's muse" , "Poetic Muse ; eighteen (anag.)" .)