Book distressing a dutiful mourner (3,6,6)
I believe the answer is:
our mutual friend
'book' is the definition.
'distressing a dutiful mourner' is the wordplay.
'distressing' is an anagram indicator.
'a'+'dutiful'+'mourner'='adutifulmourner'
'adutifulmourner' with letters rearranged gives 'OUR MUTUAL FRIEND'.
(Other definitions for our mutual friend that I've seen before include "you and I know and like" , "Novel" , "John Harmon" , "someone we both know" , "China we share" .)