N is for Noose by Sue Grafton

This is one of the Kinsey Millhone Mysteries in the alphabet series.  

Kinsey is a private investigator.  She is asked by recently widowed Selma Newquist to find out what was troubling her husband Tom before he died. Tom was a local detective who had died from a heart attack.  His death was not considered to be suspicious.  

Kinsey starts asking around and the locals all close ranks against her, and anyone who will talk to her has nothing bad to say about Tom.  Kinsey discovers that Tom was looking into some old, but not closed, cases.  She looks through all his papers to try to find clues, but within all the paperwork she can not find his most recent pocket book.  On the verge of giving up, she finds a trail to something Tom had been investigating for a while.  

In this series I have already read: 
A is for Alibi
B is for Burglar
C is for Corpse
D is for Deadbeat
E is for Evidence
F is for Fugitive
G is for Gumshoe
H is for Homicide
I is for Innocent
J is for Judgment 
K is for Killer
L is for Lawless
M is for Malice
O is for Outlaw
P is for Peril
S is for Silence
T is for Trespass

I also have the following books on my bookshelf:
Q is for Quarry
R is for Ricochet
U is for Undertow

Books with the letter V, W, X and Y are also now available in the series.