I've read lots of his books, and they're great. I don't think I would call them 'horror', though they are generally about angels and demons interacting with the mortal world, prophets discovery their power in modern times, and that sort of thing.
I think of 'horror books' more as being blood thirsty and gory, and I don't think his books are particularly either of those.
They're more, spiritual, putting christian and biblical ideas into modern day settings. Like, characters going about their business, not aware that invisible guardian angels are beside them, protecting them against the bad guys, who have invisible demons beside them, trying to get at the good guys. If you get what I mean. Or people praying, and the angels become empowered by the prayers, and defeat the demons, and that sort of thing. I quite liked the books I read.