Rupert Axebanger Brookstanton was a violent and renowned wizard who lived in England in the twentieth century. At some point during or after his lifetime, information about him was recorded in a book of sorts.
Hermione Granger read about him while trying to find information regarding Horcruxes and, more specifically, the mysterious wizard who signed himself as "R. A. B.", but could find no connection to Voldemort and thus knew Brookstanton was not who she was looking for.