A weighty book, in my opinion (4)
I believe the answer is:
tome
I believe this clue is a double definition.
'a weighty book' is the first definition.
(I've seen this before)
'in my opinion' is the second definition.
(to me)
(Other definitions for tome that I've seen before include "Large and scholarly book" , "A hefty book" , "Large, weighty book" , "Large scholarly volume" , "War and Peace, say" .)