Vast majority of chess books have chess diagrams with side to move specified by white/black square or triangle or by W/B letters next to diagram. However in some books this is missing - you can figure it out only by reading the text and I find it really frustrating. Just 2 examples: 100 Endgames You Must Know by de la Villa and ChessCafe Puzzle Book 1 by Muller. Otherwise two excellent books. Is it a lack of proper editor's work? Or is there some deep idea hidden behind?