hi iSeeLT,
glad you liked the game! that puzzle is pretty confusing.
to explain the puzzle: the shoes drawn in the book hints at you to look at the shoe lockers. it is hard to notice, but the positioning of the shoes in the leftmost locker is nearly identical to the book placement in the rightmost shelf in the library. there is one extra book on the shelf (the red book), and it is at that position at the lockers that you were supposed to click to find the key. (*•̀ᴗ•́*)و ̑̑
Wate͎̝̦̮̮r͉͓̭͔̭p̙o͖̤u̯̝c̼̘̣̞h͕͉̹̮͈͉̦'s͇͉̥̰̝ ͕̣̥̹̙s̟̳l͈͇̎̔ͤͯ͂ͮ͋a̷̦̘̺͓ͦ̍ͣ̄̏̆͗v̥̼̻̈̾̓̈́͐̐ͭ͜e̺s͖̤̄͆͆̍ͮ̍̅͝.̘̥̼̲̯̻̋ͨ