It's no spoiler to say that the killer doesn't get caught - at least not in the pages here. Joseph James DeAngelo, 72, who was fired from the Auburn Police Department in 1979, was arrested last April after DNA evidence linking him to multiple crimes associated with the Golden state Killer was uncovered; but this account of the man who was responsible for 50 rapes and ten murders from 1976 to 1986 in California - is compelling reading nevertheless.
David Fincher's film Zodiac and James Ellroy's autobiographical My Dark Places prove that lack of resolution is no barrier to a great true crime retelling yet the impact of I'll Be Gone In The Dark - a phrase the killer said to one of his victims in an early attack - is further complicated by the sudden death of McNamara in 2016 - before the book was finished (it was completed by her researcher Paul Haynes and colleague Billy Jensen from McNamara's voluminous notes).
While they do a fine job - a sense of what might have been hangs over this and one misses the verve and polish of McNamara's prose which shines in her completed chapters.
Early in the book she details leaving a glitzy Hollywood party - her husband was comedian Patton Oswalt - when she gets word on her phone that another killer she had been obsessed with has been apprehended - "I return not to my sleeping infant but my laptop... in search of information about a man I'd never met, who murdered people I didn't know".
McNamara's obsession with killers led her to insomnia, nightmares and an over-dependence on medication which, in part, caused her death.
Apparently her killer fascination stemmed from a 1984 murder near her Illinois home when she was fourteen.
She walked to the abandoned crime scene a few days later and collected the victim's broken Walkman.
But one suspects the roots go deeper than that.
Indeed it's McNamara's own story - glimpsed in tantalising fragments - which is the unresolved mystery here; a fraught home life, a rebellious teen, that increasing fascination with the darkest sides of human nature, which led to her setting up the truecrimediary.com website before narrowing her focus to the Golden State Killer.
McNamara was well aware of her obsession's toll on her health and family but, like an addict couldn't stop, even as many of her suspects in IBGITD became dead ends - "...the monsters recede but never vanish. They are long dead and being born as I write".
No doubt the dramatic possibilities of this dual mystery played a part in HBO recently picking up the rights to make a docu series.
Post arrest - it's clear McNamara was on the right track - her hunch that the perp had military and/or police training was correct, as was her belief that science would catch him in the end.
The tragedy is that she didn't live to see it.
I'll Be Gone in the Dark: One Woman's Obsessive Search for the Golden State Killer.
(Faber & Faber $32.99)