Info books about Diaries

Hi to everyone,
i'm an italian student and i'm preparin my university  final paper,  
it will be about
Diary and blog and the connection\analogies etc between the 2, do you  
know
any book about "Diary"? i keep on findin a lot of books/diary like  
"prison diary of...."
or "diary of that..." but i couldnt find any book on "diary" in  
general (i.e. how the
personal diary began, when etc etc).
thank you in advance and sorry if my question might be a lil offtopic

Michele from Palermo (italy)

ps i'm interested especially in english  books 

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