What is your all time favourite book (or books) and why?

For no other reason than I am nosey and also love books, what books do you find yourself returning to again and again? Since I find it impossible to choose (bit like naming your favourite child) if there is more than one book that you adore, that’s fine. Why is or are they your favourite books though? What is it about the story or the characters that touches you or horrifies you? I was going to list some of my favourites, but I would really rather hear about yours and why we should read them and love them too.