Quintessence of the final irrational number book (7)
I believe the answer is:
epitome
'quintessence' is the definition.
(I've seen this in another clue)
'the final irrational number book' is the wordplay.
'final' indicates one should take the final letters.
'irrational number' becomes 'pi' (pi is a kind of irrational number).
'book' becomes 'tome' (a tome is a book).
The last letter of 'the' is 'e'.
'e'+'pi'+'tome'='EPITOME'
'of' acts as a link.
(Other definitions for epitome that I've seen before include "Essence or embodiment" , "Abstract" , "Something embodying perfectly a stated quality or idea" , "Exemplar - embodiment" , "Archetype, typical example" .)