The third book that covers the trilogy tells the story of the beginning of the war, and how the battles between the werewolves known as Lycans and the vampires known as Death Dealers came into being. This book is a novelization of the film, written by Greg Cox who has written the other books. It follws the film, and as such is a good read because this franchise of horror films are very good indeed.
The story tells of Lucian, the first werewolf able to transform between his human shape and that of a Lycan, and the clandestine love affair he starts with Sonja, the vampire daughter of the underworld's tyranical leader, Viktor. Of course, things don't go as planned for the lovers, and in a world where the Lycans are laves of vampires nothing could ever possibly go well in a relationship between the two. from these catastrophic happens is born a brutal war between the two species, that spans centuries and is covered by the films and books Underworld, and Underworld Evolution.
Cox does a good job describing the world the film's have shown us. It's a book well worth reading.