Hmmm. This should be enjoyable. It's been awhile since I've been allowed a copy-cat murder.
The Count of Monte Cristo by Alexandre Dumas- it's a very good book. I do suggest you read it.
As for this death...
Brucine in lemonade should prove the best course of action. I would use
Strychnine, but I'm afraid it might kill him a bit too quickly. How enjoyable it is to do a proper book assassination again. One of my... small joys in life.
My dear Sir, he will truly suffer some of the most excruciating pain imaginable before he dies. I hope it suits your tastes.