Book published that is without question catering to niche market (8)
I believe the answer is:
boutique
'catering to niche market' is the definition.
(boutique can mean specialised or niche)
'book published that is without question' is the wordplay.
'book' becomes 'b' (abbreviation for book).
'published' becomes 'out' (I've seen this in other clues).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'question' becomes 'qu'.
'ie' placed around 'qu' is 'ique'.
'b'+'out'+'ique'='BOUTIQUE'
(Other definitions for boutique that I've seen before include "Trendy shop" , "Fashionable clothes store" , "Small specialist shop" , "shop offered" , "Small, expensive shop" .)