Amount of space in book (6)
I believe the answer is:
volume
I believe this is a triple definition!
'amount' is the first definition.
(I know that volume is a type of amount)
'of' acts as a link.
'space' is the second definition.
(I've seen this before)
'in' is a link.
'book' is the third definition.
(synonyms)
(Other definitions for volume that I've seen before include "Intensity of sound, in a book?" , "Book - measure of loudness" , "work" , "The bulk" , "Cubic capacity" .)