I can't possibly choose just one great reference book - books published by Cambridge or Oxford University Press for foreign teachers of English are all excellent.
These two books by the German author Erich Kastner were my childhood favourites. I still love the humour in the second book (I don't know what the main character is called in English - Pippi perhaps and her best friend is Tony probably). I also often dreamed of finding my lost twin just like the girls in the first story. Twin sisters separated by their parents' divorce meet at a summer camp and then change places which leads to several funny adventures.
My bookshelves will probably always be a mess - I manage to find most of the books I look for , but from time to time something disappears God knows where.