"It’s a collection of particularly dark spells most likely to be found in the homes of old families, like the Malfoys, or perhaps Yaxley Manor.  I’d not be surprised if there was one at Grimmauld Place as well, though I doubt very much that Potter would have the first clue what it is."  He made a face.  "Be careful when dealing with that book, Hermione.  Most of those spells will not even make it into the repertoire of the average Death Eater.  It’s very dark stuff."