My recommended Simmons novels would be:
Song Of Kali
World Fantasy award winning novel that takes existential terror, mixes it with the culture shock of an American in India and then tosses it at a hallucinogenic experience that may or may not be real.
The Hyperion Cantos
Published in two parts as Hyperion and The Fall Of HyperionThe award winning SF epic that begins as a new Canterbury Tales and ends as a dark tale of rebellion against a fair-faced foe. In between we get tales of love, of loss, of faith, and of epic poetry. Wonderful, wonderful stuff. Two further novels take the story forward, Endymion and The Rise of Endymion.
Phases of Gravity
After you have walked on the moon, what next? Non-genre, and a highly compelling story of a man's search for the peak experiences that give his life meaning. The story is told in the anticipation and the aftermath - never at the peak.
A big horror novel. Beginning life as a novella in Omni, Carrion Comfort is a story of vampirism without vampires, and the ultimate horror at the heart of the 20th century. It's a story of the callousness of boredom, and the redemming power of novelty.
The Crook Factory
Lying somewhere in the indefinable space between literary bigraphy, secret history and thriller is the X that marks this book's spot. An FBI agent is sent to keep an eye on (and control, if possible) Hemingway's Cuban counter-intelligence games - only to find that Hemingway is right, and things are vastly more compliated than he expected. Simmons builds his story by mixing little known episodes from Hemingway's life with the rivalries between the various secret services in the early days of the Second World War. Keep an eye out for a certain British spy...
The Hollow Man
The best SF novel about telepathy since Dying Inside. Poignant and moving, it is one of Simmons' best works. Or at least it would be if it wasn't for the pointless serial killer sub-plot...
Simmons' is a prolific writer and there's lots more out there. I've yet to read any of his ghost stories, or the vampire novels - and I've just discovered a Hawaiian horror novel that I'm going to have to track down soon...
And of course his latest SF work, Ilium, is on the to-be-read bookcases...