women have been writing love letters to ivan milat for years , book reveals .
` why they write to me is a puzzle , ' milat says .
some letters are from a woman he was acquitted of raping at knife-point .
milat brutally murdered seven backpackers between 1989 and 1992 .
he is serving seven consecutive life sentences at goulburn supermax jail .
he writes up to twice a week to his eldest nephew , alistair shipsey .
shipsey has used 94 of milat 's letters in a new book about the killer .