I am reading his latest one now--Night School, and as I was reading it, it suddenly occurred to me that it might be a good recommendation for English learners, provided of course that they enjoy thrillers.

Child is, in my opinion, a good writer who writes correct English. Furthermore, the dialog of most of his characters is informal but "good English," too. He is a wonderful storyteller. There is a certain amount of cultural context, but he makes most of it clear. His writing style doesn't seem "dumbed down," but his sentences are short, staccato, and don't contain complicated grammatical structures. 

The book opens:

In the morning they gave Reacher a medal, and in the afternoon they sent him back to school. The medal was another Legion of Merit. His second. It was a handsome item, enameled in white, with a ribbon halfway in between purple and red. Army Regulation 600-8-22 authorized its award for exceptionally meritorious conduct in the performance of outstanding services to the United States in a key position of responsibility. Which was a bar Reacher thought he had cleared, technically. But he figured the real reason he was getting it was the same reason he had gotten it before. It was a transaction. A contractual token. "Take the bauble and keep your mouth shut about what we asked you to do to for it." Which Reacher would have anyway.

The trickiest grammar is "Which Reacher would have anyway." The trickiest idiom is "a bar Reacher thought he had cleared." The Army regulation is written in formal, stilted English but you don't need to understand it perfectly.

To be clear, Reacher is a sort of superhero, and there is a fair amount of violence in the book. Reacher's guesses always turn out right, and whenever he is in a fight--three against one, six against one, eight against one--he always wins. Child is so skillful I can suspend disbelief and enjoy the story.