This is not actually an original novel by Yarbro, but rather a "novelization" of an original screenplay by John McTirenan. Many times a novelization will appear in print before the film is completed because the parties involved will provide the novelist with a screenplay sometimes even before filming begins. As a result, many novelizations feature scenes which were later cut, either during early preproduction, as the film was shot, or in post production on the editing floor.
As for the book itself, it is written with amazing clarity for such a surreal and complicated subject. Yarbro brings to life the inner world of the main character Jean-Charlse in a way the film "Nomads" did not or could not.
And overall Yarbro provides a much meatier and more easily understood narrative than McTiernan's screenplay. But then, this is the nature of the novel. The novel can enter, disect, revel and relish details and thoughts and emotions the way a 90 minute film can not.
If you are a horror fan you won't be disappointed in this book, and if you are a fan of the film you really should read this novelization, it is superb.