I ummed and ahhed about what rating to give the third book in the Necroscope series. I prefer books 1,4 & 5 to this, but the thing is that it's different and therefore commands interest in its own right.
Taking a strangely scientific almost alternate reality/sci-fi aspect, the third book goes into some detail of how the vampires came about. To be honest, the first time I read it I didn't like it, but on re-reading, my opinion of it rose considrably.
The book is fascinating on several levels, and sets up a number of future novels, including an entire spin-off series. A solid instalment in the series.