Dr. O by Robert W. Walker

Dr. Maurice Ovierto was proud of his handiwork. He removed his little trophies with great care from his victims, wrapped them in surgical gauze and sent them by Express Mail only. Human flesh was much to delicate for regular mail...
He wanted them to arrive in good condition for FBI Inspector Donna Thorpe. She alone understood his special talents, despite the pigs she worked with. He'd killed her partner to make her notice his genius, Now, it was time for her family. Soon Dr. O would have his greatest triumph: Her pain.