The 3rd Cormoran Strike novel, and the 2nd that I've read in the last week :)
From the Blurb:
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.