Black Shadows by Simon Swift:

Just when you might think: they don’t write ‘em like they used to … an author comes along with a true passion for classic rat-a-tat-tat, shoot ‘em full o’ holes, gumshoe crime fiction.
The prologue opens on October 23, 1935, in Newark, New Jersey, and grabs the reader’s attention right from the start.
Four men, operatives of The Shadow Man Detective Agency, sit around a table in a restaurant. Their work comes predominantly from the mob. The intended peace of their meal is shattered by a volley of gunfire. It’s a gangland hit, but the quartet are bystanders, not the targets, until one of the detectives returns fire and turns the hit into a free for all. In the moments of calm that follow the chaos, after seeing two of his colleagues killed, Errol Black decides to get out of that line of risky business and stick to good, honest, morally ambiguous P.I. work. However, his partner in sleuthing, Dyke Spanner, can’t resist the big bucks and stays with the mob, a decision which ends his association with Errol.
The story then jumps forward a decade to Manhattan. Errol now runs a new detective agency with another partner, Hermeez Wentz, but the business has fallen on hard times.
Errol is demoralized and the future looks bleak. Then a beautiful femme fatale walks into his office and provides him with a fresh and sordid case to work on.
Will her arrival bring prosperous good fortune, or is she there to make matters worse?
The case should have been easy money, but what starts off as a by-the-numbers surveillance job turns deadly.
Factual elements of the plot are skilfully weaved together with fiction to create an entertaining and wonderfully nostalgic detective story in the tradition of the hard-boiled noir classics.
When considering a career path in life, follow the advice of Arthur Flegenheimer in the prologue: “Steer clear of wise guys, they’ll kill you!”