Where to find the secret items for the 3 unannounced games and what the effects of doing so are...
Book 1/4 as soon as you enter the main hall after picking up the guest pass, at the bottom of the stairs to your immediate left.
Teacup 1/4 Upstairs in the Carrion room.
Bag 1/4 to the right of the Weird West room's entrance
Teacup 2/4 On the side of the Weird West room
Book 2/4 directly in front of you after riding the small elevator up to the floor above
Teacup 3/4 On the table of the room where the credits are playing
Book 3/4 A dead end in the vents before falling outside into the next hall
Book 4/4 On a table in the Blightbound room.
Bag 2/4 Hanging off the side of the harpoon for the Olija room.
Bag 3/4 near where the serious sam trailer is playing
Teacup 4/4 Fall guys upstairs section, to the left of the trampoline
Bag 4/4 on the statue as you are leaving the Shadow Warrior room
Effects of Finding Everything
After you collect a set of secret items it will change up to 3 of the pictures on the website page after beating the boss and leaving: