in the books:
-Rita is alive and became pregnant after the marriage.
-La Guerta is dead (last victim of the Icetruck Killer, who isn't actually called that way)
-Dexters Brother escaped
-Dex trains Astor and Cody, due to them showing sociopatic tendencies thanks to Paul
-the Dark Passenger is actually some kind of demon
-Deb has known Dexters secret ever since Brian had kidnapped her (she was awake during their conversation) and she doesn't rat him out. She also has big boobs.
I've always thought about reading the books but am going to pass now, they sound crap compared to the TV show.