Why we consult reference books of course (2,2,4)
I believe the answer is:
to be sure
'of course' is the definition.
(thesaurus)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for to be sure that I've seen before include "Undoubtedly" , "Admittedly" , "Of course, certainly" , "I admit" , "Without doubt" .)