Despite the fact that all three of those books utilize suicide as a major character point, I didn't feel like the tone of any of those books was particularly dark. In fact, there were more moments in those books that felt downright saccharine to me. I think that an author's writing style and ability to convey the realities of human emotion are key when it comes to my own emotional experience with a book. I've read other books with less overtly "dark" subject matter that left me feeling far more morose.