Magick Moste Evile is a Dark arts reference book, written in the early middle ages by Godelot (TBB).
It refers to Horcruxes in its introduction, but only to call them "the wickedest of magical inventions" and to say that the book will not speak of the subject further. At that, this book was the only book Hermione could find in the Hogwarts library, including the Restricted Section, that referred to Horcruxes at all; that particular copy let out "a ghostly wail" when Hermione slammed it shut in irritation (HBP18).
More than twenty years later, Hermione has a copy of Magick Moste Evile on her bookcase in her office as Minister for Magic (CC1.19).
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Tags: books curses evil library books old books