The Witch's House is a series of books found in the 2nd Floor Library. Each book provides a hint regarding the house itself, the Witch residing in it and her Demon.
Initially, only "The Witch's House (1)" and "The Witch's House (2)" will be present when you first enter the library. But as you progress and read certain diaries, additional books will appear.
The Witch's House (1)
"Keys do not open doors in the Witch's House. Something else must serve as a key."
This book most likely clarifies the reason why the Silver Key (given to the player by the Chef) does not work on the door to the Second Floor.
The Witch's House (2)
"The form of the house changes based upon the Witch's magic."
The Witch's House (3)
"The Witch's House devours humans. Their souls are offered to a Demon."
This book can only be unlocked when the player successfully explores the "no distraction" hallway, and reads the diary (and other texts) found in it. As long as you don't read the note before the "no distractions" hallway, you're free to go to the sides.
As of 1.07, your reading of the warning is remembered across playthroughs. Delete PublicData if you want it to forget, though this will also clear your progress in the game.
The Witch's House (4)
"The Witch's House is purposeful. If any spirits residing in it remain unfed to the Demon, that purpose is unfulfilled."
The purpose of the Witch's House is to feed the souls of its victims to the Demon. However, according to the Diary of Ellen, the Demon doesn't always devour human souls entirely. The spirits residing in the house are actually remnants of said souls, making the house's purpose remain unfulfilled.
As the player progresses in the game, these spirits eventually disappear or die out after encountering them. It can be theorized that after fulfilling their original purpose to help Viola, they are later devoured by the Demon.
The Witch's House (5)
"When the Witch's flowers wilt, the Witch dies."
This volume refers to the witch's roses. In the Diary of Ellen, Ellen states that, to a witch, the roses are like her flesh and blood, as the petals seemed to bear patterns like blood vessels that were nearly identical to the veins on her hands. As the roses seem to represent a witch's vitality, destroying them would have a similar effect on her body.
Moreover, for every victim killed by the witch, a new rose will bloom.
This may also account for the huge roses that blocked the player's exit from the forest.
The Witch's House (6)
"The Witch's House is magic, granted to a Witch by a Demon."
The Witch's House (7)
"Once the Witch shows herself, the House returns to normal."
When the player first enters the Witch's House and visits the 1F Present Room, they will come across a closet that says "Opens when the House returns to normal". This book hints that the player can only open the closet and retrieve Ellen's Knife during the chase.