I read his books in German, but there seem to be English translations and unless they’re really bad it should not matter for enjoying his writing or the stories. I read Noah first, but as far as I remember it’s a very good thriller, but not that psychological, at least in comparison. After that I just started his books chronologically. Does not matter though, there is no continuing story line or returning characters. But I can vouch for the psychological character of at least his first four books (Therapy, Amok, The Child, and The Soul Breaker).

I you like psychological horror/thrillers then Sebastian Fitzek will keep you busy (and maybe awake at night)