Decline English books (3)
I believe the answer is:
ebb
'decline' is the definition.
(I know that ebb is a type of decline)
'english books' is the wordplay.
'english' becomes 'e' (abbreviation).
'books' becomes 'bb' (two abbreviations for book).
'e'+'bb'='EBB'
(Other definitions for ebb that I've seen before include "Outward flow of the tide" , "Not to wax" , "Initially Mrs Browning's" , "(Water) recede" , "Drain away; decline" .)