Brian Lumley is a British horror author whose works generally fall into three categories; Vampire Horror, Lovecraft-inspired Cthulhu Mythos based horror and fantasy (which I'm yet to get around to). His writing tends to be very descriptive and inspires a sense of dread.
After years of reading horror, Brian Lumley was the first author since Stephen King's Pet Sematary (and the first as an adult) to make me feel ill while reading a horror novel. I felt so disgusted that I said to myself - "I'll give this book 20 more pages to improve and if it doesn't, I'm putting it down".
It did improve and became my favourite novel of all time (so far). A position it has enjoyed for more than 15 years on my list.
It's probably also worth mentioning that George Underwood's amazing covers really helped to sell the books to me.
The Necroscope Series (Esp 1,3 & 5) is absolutely incredible, riveting and epic reading. Imagine you could speak to the dead, and you found a vampire.
Khai of Khem does what Stargate should have done, several years earlier. It's pretty freaky in parts.
The Psychomech trilogy is pretty special, horror, violence, technology and Nazis. What more could you want?
Anything in boldface is still outstanding (I haven't read it).