The main characters are stellar. This is my first Hiro Hattori novel, but it is the fifth in the series, and it didn't bother me a bit. A shinobi, Hiro is tempered like steel; quick-witted, too. His companion, a Portuguese priest called Father Mateo, is exactly how I remember the priests I grew up with in church: thoughtful, often cheery, and warm if not a little headstrong. Their interactions are written so beautifully that through them you get to know these men. And, like all good detective novels, they'll feel like old familiar friends by the end of the story. Based on that, I wouldn't worry as to where you enter into this particular novel set.
I've read mysteries, and I know those, but the setting here was entirely exotic and unfamiliar. And deadly. Taking place in Iga, in a village full of trained assassins and quiet warriors, it's impossible to point fingers without having them cut off.
That level of danger is palpable on every page of this book, to the point where when a character ventures out into the dark with only a lantern, it will make you shiver. To make it worse (because in these books, you have to make it worse) this is Hiro's family village, and many implicated in the mysterious murder of a clan representative are his own loved ones.
The details are everything, which is to be expected from a historical writer, but really the characterization here was just amazing, and it's that which will transport you across the world and across time into this beautiful, dangerous setting.
I'll be looking for the rest of these books by Susan Spann, and I'd recommend "Betrayal At Iga: A Hiro Hattori Novel" to any mystery lover.