This is a frustrating mix of good ideas, interesting details, cardboard characters and horrible prose.
It deserves credit for what was at the time a fairly fresh take on vampire fiction. It's heavily influenced by Interview with the Vampire, in that the vampires have a secret society with roots in ancient Egypt, they aren't dead but instead hyper-alive, and they can pass on their powers by giving you some of their blood; but where Rice (until later books) doesn't really try to explain any of this, Strieber puts a science-fiction spin on it and puts the vampires up against a human who might actually figure out what makes them tick.
Unfortunately, that's also the biggest problem with the book. The middle third or so is almost entirely devoted to a research project, with pages and pages of technobabble and academic politics. If I had read this in the early '80s as a young fan of horror and technothriller paperbacks, I might have enjoyed this combination of the two genres for its surface shininess; Strieber does know how to get some pulp energy out of beats like "This blood isn't human!", "These brainwaves have extra high voltage!", etc. (He also deploys all the standard pseudoscience truisms, e.g. quantum theory proves that synchronicity is real, and you don't use most of your brain.) But ultimately none of this stuff matters—the scientists don't actually discover anything that will either help or hurt the vampire. And it makes the vampire look incredibly stupid, since her decision to let them study her makes no sense at all for someone who's spent 3500 years trying to avoid human attention.
But the rest of the book is about other things. There's an extensive backstory for our lead vampire Miriam, in various places and eras, trying to survive various threats while she keeps losing her chosen human companions—the people she vampirizes, unlike her, aren't really immortal. This is pretty interesting, probably involved a lot of research, and is also extremely gory. It doesn't give any particular depth to Miriam as a character, though; all we really know about her is that she would like to have a longer-term relationship, and that like all vampires, she's obsessively concerned with security (they're so determined to live forever that, in the present day, they'll only drive cars with the highest safety ratings).
The present-day storyline is about Miriam's life with John, her current ex-human consort; John's discovery that his lifespan is about to run out; and the head of that research project, Sarah, who catches Miriam's eye first because she might be able to cure John, and then because she might replace him. It's a good idea for a triangle (which became pretty much the entire focus of the movie adaptation) but there are a couple of problems here too. First, John's and Sarah's stories never intersect in any way; Strieber is so determined to keep them apart that it becomes unintentionally funny how close they can get without meeting. Second, John has basically no identity outside of his life with Miriam, while Sarah, until very late in the book, doesn't even get her own point-of-view narration—she's described either by Miriam, or by her dim boyfriend Tom who thinks things like "Sarah's miracle was the purity of her womanhood." Sarah also has virtually no agency: once Miriam decides to woo her, there's pretty much nothing a mere human can do (and unlike the movie, this is portrayed as not in any way consensual; Sarah is 200% straight, and Miriam even molests her in her sleep). So, rather than three people pursuing various goals, we basically have one person who has a chance and two who have no chance.
This is interspersed with lots of murders. Strieber clearly put some thought into how a vampire would resemble an organized serial killer, but apart from the verbose details of how they picked the lock on a house or whatever, these all kind of blur together into a lot of "Yes, we must kill people, because that's our way, and also they taste really good." There's one of these that should be particularly upsetting for several reasons (this was in the movie), but it's forgotten about very quickly. There are also some extremely clunky sex scenes; I'm going to generously imagine that the awkwardness of these was intentional, as the book often seems to take Miriam's point of view that human relationships are stupid.
It ends in a way that makes the rest of the book almost entirely irrelevant, and leaves things open for a sequel, which he apparently wrote a couple of.