Book space (6)
I believe the answer is:
volume
This clue is a double definition.
'book' is the first definition.
('volume' can be a synonym of 'book')
'space' is the second definition.
(I've seen this before)
(Other definitions for volume that I've seen before include "Gross < sound" , "The bulk" , "Space occupied by an object" , "size" , "Publication" .)