Name of the book I'm writing (3)
I believe the answer is:
kim
'name' is the definition.
'of the book i'm writing' is the wordplay.
'of' says the answer is hidden in the clue.
'KIM' is hidden in the letters of 'book im writing'.
(Other definitions for kim that I've seen before include "-- Basinger, US actress" , "Boy's or (now usually) girl's name" , "Rudyard Kipling novel" , "Girl" , "Book" .)