Hmmm...
All of them!
That is true - all of them that I've read have influenced me in their own ways. However there are a few that stand out in my mind as writers that gave me inspiration to continue writing and kept instilling the love of the genre.
Those are (in no particular order) -
Dean R. Koontz
Robert McCammon
Ramsey Campbell
William Hill
Peter Benchley
Charles Grant
Brian Lumley
Thomas Harris
Of recent note (ones I've discovered in the past 5 years) -
Scott Johnson
Steve Alten
Gary Braunbeck
Darren Franz
Rick Reed
James Herbert
Michael Crichton
To be fair I also have to say I've been influenced a lot by the thriller and suspense writers. In fact, when you read Apparitions you'll likely notice that although it's tone and content is most definately horror it's written in a suspense novel style. The same holds true with Night Wing - it's fantasy written like suspense.
I read in all genres though so the grand total of all those fine authors has played a part. From western writers to sci-fi, from fantasy to horror, from romance to suspense.
