diarist:
(plural: ‘diarists‘; pronunciation: the letters ‘di’ rhyme with the word “die” — for the right pronunciation, please click here)

former American President Harry S. Truman, a famous diarist
a person who regularly writes down the incidents/happenings, often including his/her personal feelings and opinions on certain issues, in a ‘diary’; a diary keeper;
a person whose daily events and opinions on those events have been recorded and published;
a journalist,

modern day diarists using modern gadgets in place of diaries (paper books)
I still remember our Crafts teacher telling us to maintain a small pocket diary and note down the events that we experienced every day, and every month, giving one of us the ‘best diarist’ award for writing down most events in his diary.
Though a diarist is a careful person who writes down his/her opinions and feelings on the daily events and keeps the diary very safely, some times this innocent act of writing down one’s very personal feelings may prove to be harmful because once the so personal feelings on others are exposed by any curious friend or relative who has access to their diary, the diarist finds his/herself in an embarrassing situation or, in some cases, in a very dangerous situation!
Biographers find it easy to write biographies of diarists very easy because most of a diarist’s day to day activities and the opinions are found in their diaries and can be used as reference or authentic proof.
