Masses of books (7)
I believe the answer is:
volumes
I believe this clue is a double definition.
'masses' is the first definition.
(volume is a kind of mass)
'books' is the second definition.
('volume' can be a synonym of 'book')
'of' acts as a link.
(Other definitions for volumes that I've seen before include "Parts of periodical publication" , "encyclopedia?" , "Books (in a series)" .)