From the Super Mario Wiki
Bowser's Diary is a diary belonging to Bowser that is seen exclusively in Paper Mario. It can be found while playing as Princess Peach after rescuing the first Star Spirit, Eldstar. The diary is found in the room containing the secret passage to Peach's room. After Princess Peach sneaks out, she and Twink find this diary sitting on a desk. Twink reads the diary for Peach, revealing the location of the second Star Spirit, Mamar. Suddenly, Bowser enters the room, realizing that he had left his diary out. After discovering that Peach has snuck out and was reading his diary, he becomes angry, and orders two Koopatrols to take her away. After this, Bowser removes this diary, and it is not seen for the rest of the game.