Why the diaries are used on the vampire diaries,
Elena's Diary: From the day her mum gave her her first journal Elena was push in the direction of being a journalist by her mum , Ever since she died Elena kept her diary and kept writing in it about her life ,Vampire ect..
Stefan's Diaries:Since Stefan is a vampire he has a lots of diaries from back in the day, help from his best friend Lexi Stefan regand more or less his humanity he started writing in diaries to express his emotions feeling thoughts ect .Being ripper Stefan made him "forget " or "ignore" is diaries and when he threatened Elena off the wikery Bridge he started trying to write again.
Grimoires: as once said by Stefan Salvatore ,"every spell a witch casts is unique apon it self"' that mean every spell a witch casts is writen on these grimores because every spell is unique in every other ways an other spell cant be.
Gilbert journals:Since Johnaton Gilbert was a founder member of mystic falls every founder had wooden bullets ,safe hidden hidding spots and best of all , every council member had their own journals, written in them are the best ways to kill, capture and poison a vampire.
Mason's Diary: Mason's Diary is about his first full moon ,and what he did to weaken him self (drinking wolfsbane) he explans what he goes through and how much time it took to break every bone in his body and how long he was a werewolf for