Career of Evil by Robert Galbraith. If you didn't know already, this is a pen name for J.K. Rowling.
This is the third installment of the series following the exploits of Cormoran Strike.
Here is the description from Amazon:
When a mysterious package is delivered to Robin Ellacott, she is horrified to discover that it contains a woman's severed leg.
Her boss, private detective Cormoran Strike, is less surprised but no less alarmed. There are four people from his past who he thinks could be responsible--and Strike knows that any one of them is capable of sustained and unspeakable brutality.
With the police focusing on the one suspect Strike is increasingly sure is not the perpetrator, he and Robin take matters into their own hands, and delve into the dark and twisted worlds of the other three men. But as more horrendous acts occur, time is running out for the two of them...
Career of Evil is the third in the highly acclaimed series featuring private detective Cormoran Strike and his assistant Robin Ellacott. A fiendishly clever mystery with unexpected twists around every corner, it is also a gripping story of a man and a woman at a crossroads in their personal and professional lives.
--I love this series. I expect movies soon.