Great example of mathematical ratio in e-book (7)
I believe the answer is:
epitome
'great example' is the definition.
(as in the epitome of something)
'mathematical ratio in e-book' is the wordplay.
'mathematical ratio' becomes 'pi' (the number pi).
'in' is an insertion indicator.
'book' becomes 'tome' (a tome is a book).
'e'+'tome'='etome'
'pi' inserted within 'etome' is 'EPITOME'.
'of' acts as a link.
(Other definitions for epitome that I've seen before include "Something embodying perfectly a stated quality or idea" , "Exemplar - embodiment" , "Summary; embodiment" , "Abstract" , "Model" .)