From the Super Mario Wiki
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Bowser's Diary is a diary belonging to Bowser that can be found in Paper Mario while playing as Princess Peach after rescuing the first Star Spirit, Eldstar. The diary is found in the room that has the secret passage to Peach's Room. After Princess Peach sneaks out, she and Twink finds this diary. Twink reads the diary for Peach, which reveals the second Star Spirit's location. Bowser then comes in, finds Peach is reading his diary, gets angry, and makes two Koopatrols take her away. After that, Bowser removes this diary and it is not seen for the rest of the game.