Books and letters providing material for test (6)
I believe the answer is:
litmus
'material for test' is the definition.
(as in a litmus test)
'books and letters' is the wordplay.
'books' becomes 'lit' (abbreviation for literature).
'and' says to put letters next to each other.
'letters' becomes 'mus' (mu is a kind of letter).
'lit'+'mus'='LITMUS'
'providing' acts as a link.
(Other definitions for litmus that I've seen before include "Substance used to tell acid from alkali" , "dye encountered in testing" , "Substance used in test for acidity" , "'It turns red with acid, blue with alkaline (6)'" , "Chemical tester" .)