There isn't one single author of a book called "Diary." A diary is a personal journal, and the author is always the person writing in it.
If you're interested in famous diaries, here are some examples:
* The Diary of a Young Girl by Anne Frank
* The Diary of Samuel Pepys
* The Diary of Virginia Woolf
Do you have a specific diary in mind? Maybe I can help you find more information about it.