Out of all the books (excluding e-books) I own, I chose these three to be deep.
The Chronicles of Narnia may fall under children's literature but the symbolism and meanings blatantly portrayed or tucked in nooks & crevices in the story are deep and hinged upon Biblical references. Aslan's sacrifices and actions mirror that of Jesus Christ especially towards the end where they.... secret! I will not spoil. Better finish the whole series. Don't rely on movies because there are lots of missing pieces there but it's good to watch them 'cause one can compare how one imagined them with how the producers imagined the story.
The Dark is Rising is, I believe, teen fiction. I read this when I was in grade school & I was so confused. I understand what happened now but the scenarios are really difficult to imagine! Probably because of all the "time traveling" they did & the Old Ones existing in the past, present, and future. When I try to remember now, it's kind of a muddle. Overall, I find this story deep and quite difficult to understand especially in the emotional aspect. Loose threads everywhere and the reader is left to speculate on the decisions of the protagonists & how they will eventually lead their lives.
This is pretty presumptuous of me but I haven't read The Return of the Young Prince yet but if it is anything close to the first book, The Little Prince, then I am already expecting it to be deep. (ㆁωㆁ*)