Religious discussions aside, the Bible is written in Old English. The language really is archaic. Some parts are incomprehensible even to native speakers. I just had a look at an English translation of the Qu'rann online, and it looks much the same. I'm not trying to offend, I'm just saying that they're not suitable for someone who wants to learn English.
What kind of genre do you like?
I know you said you don't want to read books for children and teenagers, but level o
Matthew Reilly novels are aimed at young adults, but the language is simple enough that children wouldn't have a problem reading them. I think his early novels are a lot simpler (in language) than his more recent ones though. I've seen kids around ten years old read his novels, but my dad has also enjoyed some of his novels.
Wilbur Smith is a very good and experienced writer. He's been writing for a long time. His language is a bit more novel-like than Reilly's. I know people my age that read his books, as well as people's grandparents. Most of his novels take place in Africa, and usually have some historical context. Some of his novels that take place in ancient Egypt are a bit more fantastical. I've only read a few of the many novels he's written though.
If you like horror, read novels by Stephen King.
