Books at college were illuminating (3,2)
I believe the answer is:
lit up
'were illuminating' is the definition.
('they lit up' means 'they illuminated')
'books at college' is the wordplay.
'books' becomes 'lit' (abbreviation for literature).
'at college' becomes 'up'.
'lit'+'up'='LIT UP'
(Other definitions for lit up that I've seen before include "Started smoking - being drugged!" , "was illuminating" , "Prepared to smoke" , "high" , "bulb so transformed room?" .)