Yeah, comparing the books to the TV show is like comparing the Lord of the Rings Trilogy to the Silmarilion. There are a lot of things that are condensed or glossed over for simplicity sake. Outside of the big players and their medium-sized underlings, they didn't really delve all that deeply into the politics of the various Houses and their subordinates, fiefdoms, and all of that.
I mean in the show, beyond the great houses there's the Bolton's, Umber's, Karstark's, Frey's, Mormont's, Glover's, and the Tarly's. Also people know the Clegane's, Dayne's, Dondarrion's, and Tarth's though it's really easy to forget that those characters are from actual lesser houses. That's about it.
To show you just how many more there are,
here's a list of all houses from the book (though it includes ones that have been wiped out as well as different versions of the same house), but it's still easy to see just how massive the list is compared to the show.