Hi Killjoy,
Thank you for taking the time to play our game!
We are aware that some of the puzzles are quite unclear but we didn't have enough time to add more hints /get more player feedback. As for the empty cubby hole, the intention was to have the shelf in the library with the red book to match to the shoes shelf(books at the position of shoes, red book at the position of the key).
Thank you for your feedback and we are glad you enjoyed it :)
Wate͎̝̦̮̮r͉͓̭͔̭p̙o͖̤u̯̝c̼̘̣̞h͕͉̹̮͈͉̦'s͇͉̥̰̝ ͕̣̥̹̙s̟̳l͈͇̎̔ͤͯ͂ͮ͋a̷̦̘̺͓ͦ̍ͣ̄̏̆͗v̥̼̻̈̾̓̈́͐̐ͭ͜e̺s͖̤̄͆͆̍ͮ̍̅͝.̘̥̼̲̯̻̋ͨ