I finally decided to buy a use copy of Brian Lumley's Blood Brothers, which seems to have gone out of print (as far as I can see, none of the other books in the series are). This is the first volume of the Vampire World Trilogy, which chronicles the story of Harry Keogh's twin suns, Nathan and Nestor, in the alternate universe where the vampire menace was spawned.
Both boys take after their father. Unfortunately, Nestor seems to have all of his father's worst qualities and ends up becoming Wamphyri. Nathan's a much better sort and goes around befriending people and learning. I like the characters. I like the plot (it has lots of action). I like the opening, despite its flaws.
The opening is a recap of past events, a mini-biography of Shaitan (the first vampire), and a history of how a vampire colony was founded across the desert from the inhabited parts of the vampire world. Normally, putting that kind of stuff at the beginning of a book would annoy me, but it was so well written that I didn't care.
This is probably the best book in the Necroscope series, except for the original. I give Blood Brothers a full five yo-yos.