Small tip: books tell us lots (5,7)
I believe the answer is:
speak volumes
'tell us lots' is the definition.
'small tip books' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'tip' becomes 'peak' ('peak' can be a synonym of 'tip'**).
'books' becomes 'volumes' ('volume' can be a synonym of 'book').
's'+'peak'+'volumes'='SPEAK VOLUMES'
(Other definitions for speak volumes that I've seen before include "Be eloquent; be good evidence (for)" , "Be silently eloquent" .)