It supposed to be the first 3 books Nocturnal. The studio's plan was do 4 movies, each one spanning three books because thats how the series was broken down. The books were 4 sets of 3 trilogies: Vampire Blood trilogy, Vampire Rites trilogy, Vampire War trilogy, and Vampire Destiny trilogy.
Even though the movie spans the three books of the Vampire Blood trilogy, you are right you can tell they changed a lot. They added a new love intrest for Darren (the chick with the tail isnt Debbie, but is an all new character), and it looks like they may have already blooded steve, and Darren does not have flitting and super spit in the books like has been seen i nthe trailers. So big changes.....we shall see how it turns out.