If you're looking for a page-turning thriller, I recommend "Alter Ego," by Tory Allyn. Here's the synopsis:
"After a partially-covered body displaying unusual physical characteristics was found in the backwoods of Virginia, FBI agent Jack Stanwick is once again summoned. This was the third victim to be discovered in this condition, and with most of the brotherhood on another high-profile case, he needed some outside help. Jack knew just where to go. He hires three former police officers who work for the Davenport Detective Agency. During the initial investigation, the detectives focus their attention on the militia group, M.A.G.O.C. (Men Against Government Overtaking Control), only to find that several members and a high-ranking official in the White House are involved in a governmental conspiracy. Digging further, the detectives come upon Prescott Chemicals, where an elixir that was discovered in the Amazon Rainforest by the Mayapo natives years before, is now being transformed into a formula with irreversible consequences. While investigating a lead, the detectives accidently come into contact with the elixir and experience a change to their bodies and minds causing some amusing, yet permanent issues. With the assistance of a beautiful journalist and a bitter French physicist, the detectives come upon the location of the perpetrators and uncover a plot which culminates into the betrayal and a secret that affects one of detectives, changing his life."
Here's the Amazon link: http://www.amazon.com/Alter-Ego-Tory-Allyn/dp/0692287507