The Gore Vidal novel is excellent. But if you want a definitive biography, I would agree with Jerseyman and recommend the one by James M. McPherson.
I can also recommend Manhunt, by James L Swanson. It's not about Lincoln as such, but about his assassination and the hunt for the conspirators. It's really quite gripping. I started browsing it in a Barnes & Noble, decided to buy, and finished it almost at one sitting.
Mike
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