This spellbook was given to Winifred Sanderson by the Devil himself; bound in human skin, it contains the recipes for her most powerful and evil spells.
The book has a human eye on the front cover, and often sleeps, only waking to the sound of its mistress who dotes on and cherishes it, meaning the book is alive and, on some level, intelligent.
Among its contents is the Life Potion, an evil potion that allows the witches to suck the life-force of the drinker to replenish their youth. Other spells include cursing a vicim with immortality, transforming victims into cats, raising the dead, cursing victims with boils and blisters, etc.
It's most important curse was that if in the event the witches were killed, that if a virgin lit the Black Flame Candle on Hallow's Eve during a full moon, the witches would return alive for one night unless they suck the life out of a child lest they turn to dust come sunrise.
The book is protected by magic and cannot be burned or destroyed. It is an unholy record of evil magic and nothing good can from it. After the deaths of the sisters, the Book was last seen in the witches' cottage, opening its eye, and presumably remained there.