For exam, American books believer (10)
I believe the answer is:
protestant
'believer' is the definition.
'for exam american books' is the wordplay.
'for' becomes 'pro' (as in being 'pro' something).
'exam' becomes 'test' (synonyms).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'books' becomes 'NT' (abbreviation for New Testament).
'pro'+'test'+'a'+'nt'='PROTESTANT'
(Other definitions for protestant that I've seen before include "A Christian" , "Member of a church separating from Rome at the Reformation" , "Martin Luther, for instance" , "Lutheran, perhaps" , "Opposing" .)