Most everyone is familiar with the movie based on The Exorcist, and with the third movie, The Exorcist III, which he also wrote and directed based on his novel Legion. They are truly amazing movies; I remember being terrified in the most delightful way watching them when I was a kid. But later I read the novels and was just blown away by his writing. He had a simple, yet beautiful, almost poetic way of writing, that belied some of the horrors he was writing about. His writing was though-provoking as well as sublime, as he tackled such things as the existence of God and true Evil, as well as what it means to be human. His is the kind of writing that will stand the test of time, the kind of writing I aspire to.
So if you haven't read these novels, please do. Especially if you're a writer.
You can learn a lot in his work.