The last of his tribe of assassins, Mathayus is tasked with stopping warlord Memnon from completely taking over the known world. The only way to do it, is to kill Memnon's sorcerer. But the sorcerer turns out to be a sorceress, more a prisoner than ally and advisor to the warlord, and together, and with the help of a motley crew of outlaws, bandits and warriors, they will strive to change the destiny foretold in Cassandra's prophecy.
While the movie is your typical popcorn-y fare coupled with C-movie worthy acting in which you can send your brain on an hour-and-a-half vacation, the book expands on the world, characters and their motivations, inner thoughts and emotions.
That gives the story a few more layers and a little more depth, but that's pretty much it. It's definitely better than the movie (which isn't such a difficult feat), but still firmly in the average field.