In order to access the secret level, E2M9: Thin Ice, you must collect five tomes that are scattered throughout E2M4: The Overlooked Hotel. This will create a teleporting fog in the room where the Moon Key is found. The tomes are located either in plain sight or in secrets. No talley is provided by the game if you pick these up. however a triangle mark appears on the floor of the Moon Key room for each tome collected.
- Near the Spider Key
- Near the Dagger Key
- Behind the bookshelf in one of the bedrooms (secret)
- In a hidden room near the library (secret)
- In the library on top of a bookshelf (secret)
In the fan add-on Death Wish (2011, 2017-2018), the player can collect modified tomes that bring up messages when you collect them. There are several in each level, and they have special messages that reveal secrets, story elements, and sometimes nothing more than offhand Cultist comments. They are not always right in your path, but if a curious player starts searching every corner they should be able to find them all.