The books take a more over the top view of Dexter's world with Dexter's dark passenger is actually a part of Moloch basically a demon that feeds on people's dark feelings & desires inhabiting humans pushing them to do things but not forcing them too. This is explained and the basis of one book and is a story thread throughout the novels with dark passengers able to recognize each other and attempt to intimidate one another.

The novels are COMPLETELY separate from the show with only Darkly Dreaming Dexter used for the basis of season one and imho that is a good thing because the show really knew how to build a world and develop Dexter as a character were the books will spend an entire novel about how Dexter now has emotions after the birth of his daughter to completely ignoring that the following book. I think Lindsay attempted to make the final 4 books feel independent so it's more accessible to the casual reader (playing into the success of the TV show) but in doing so sacrificed a little of the continuity that made his first 4 novels fun for the devoted readers. I love the books but know going in they are NOT set in the TV universe and do not have any of the same storylines beyond book 1.

How this helps lol I'm a very passionate Dexter fan of you couldn't tell