-A Canticle for Leibowitz by Walter M. Miller Jr. Really good sci fi novel.
-I'm currently reading Journey to the Center of the Earth. It's a classic for a reason, good stuff.
-Breakfast of Champions
-Lots of stuff by Vonnegut. Cat's Cradle is one of my favorites. (It's the one with Ice-9 right?)
-Adventures of Huck Finn
Some of the ones already mentioned that I would also highly reccomend to anyone:
-Hitchhiker's Guide series
-I, Robot
-Dune
-Andromeda Strain and The Great Train Robbery (I'm not as in to Chrichton currently, but I have read every novel he's authored outside of maybe his most recent or two.) I'd imagine moxio mentioned them as being complex because Chrichton tends to really lay down a lot of technospeak.
-For King stuff The Long Walk and Running Man are both on the short side and fun reads.
-For an extremely guilty pleasure check the Rogue Warrior series. Basically Rambo-esque action stuff. (Okay this wasn't already mentioned

)
-Another guilty pleasure of mine are Salvatore books (fantasy stuff) although I haven't read anything by him in several years.
-And if you do like fantasy at all definitely read the LOTR books (Hobbit, plus the 3 made into movies.) The Harry Potter novels are also fun light reads. Some of them are very long, but they go very quickly.