The Limehouse Text by Will Thomas
Before taking up private enquiries for a living, Barker had lived and traveled extensively throughout Asia, working as a ship's captain at one point, marching with the British army at another time. This experience comes in handy as he tries to get to the bottom of who killed his previous assistant, the man Llewellyn replaced. That assistant had been from China and it turns out the he may have died because of his brief possession of a book from that country - the Limehouse text of the title. The book is no ordinary book - it teaches secret ways to kill.
"Dim mak" is apparently a real martial art and among the teachings are how to disrupt a person's inner workings with just a touch. In any event, this knowledge was never meant to leave the monastary it was stolen from and it certainly wasn't meant for Western eyes. The text gives Barker a second task to perform - find the killer and the book before it falls into the wrong hands.
Anyway, the characters are all richly drawn and it's great fun to watch them interact. The book is highly recommended.