Brian L. Porter 
Purple Death [EPUB ebook] 

A series of murders leads Detective Inspector Sean Connor and his team into a labyrinthine investigation. The victims are dispatched using a poison previously associated with the notorious Borgia family.
As the murders multiply at an alarming rate, Connor finds clues hard to come by, and every lead takes him down yet another blind alley. The killer seems to be one step ahead of him at every turn.
Together with Sergeant Lucy Clay, they must piece together the shreds of evidence and find the mastermind behind the murders that become known as Purple Death.