Tower of Thorns

The Irish historical fantasy buddy cop team of Blackthorn and Grim heads north to solve a bizarre mystery afflicting Bann, a little-known chiefdom plagued by a wailing monster trapped in a tower.

I seriously thought I had this book all figured out. I'll even admit I was getting bored, but then WHAM! The plot twists twist away. Marillier's other books aren't so plot heavy, and these ones don't seem like they are, but that is just her misdirecting the reader. This is a mystery, and a damn good one.