A number of books — English — penned by a single German (7)
I believe the answer is:
integer
'a number' is the definition.
(I know that integer is a type of real number)
'books english penned by a single german' is the wordplay.
'books' becomes 'NT' (New Testament).
'english' becomes 'E' (abbreviation).
'penned by' means one lot of letters goes inside another (inserted letters are penned in).
'a single' becomes 'I' (one).
'german' becomes 'ger' (abbreviation).
'nt'+'e'='nte'
'i'+'ger'='iger'
'nte' going within 'iger' is 'INTEGER'.
'of' is the link.
(Other definitions for integer that I've seen before include "Positive or negative whole number" , "The whole thing" , "Get Erin (anag)" , "Say one" , "Perhaps 60,000" .)