Book a specific couple of planes that take up gliders (3,3,6)
I believe the answer is:
the two towers
'book' is the definition.
(book by JRR Tolkien)
'a specific couple of planes that take up gliders' is the wordplay.
'a specific' becomes 'the' ('the' is used to refer to a specific item).
'couple of' becomes 'two' (a couple is two things).
'planes that take up gliders' becomes 'towers' (planes which tow gliders).
'the'+'two'+'towers'='THE TWO TOWERS'
(Other definitions for the two towers that I've seen before include "title?" , "Definitely more than one keep" .)