Kindle download (4)
'kindle download' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for book that I've seen before include "Hardback, say" , "Volume - maybe thrown at offender" , "Paperback" , "Strict rules" , "Reserve - bets taken on a race" .)
(Other definitions for ebook that I've seen before include "Publication" , "something to read" .)