An extra book paid for by you and me (5)
I believe the answer is:
bonus
'an extra' is the definition.
(bonus can mean extra or additional)
'book paid for by you and me' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'paid for by' becomes 'on' (as in 'the drinks are on me').
'you and me' becomes 'us' (the two of us).
'b'+'on'+'us'='BONUS'
(Other definitions for bonus that I've seen before include "commission" , "Payment additional to the expected or usual" , "Extra money paid for good performance" , "Sum added to salary for good performance" , "bribe" .)