From the Super Mario Wiki
The Ragged Diary is the diary that belongs to Ghost T. in Paper Mario: The Thousand-Year Door. Ghost T. lost it on the Excess Express and never found it. Ghost T. later died and still didn't find it; he was scared that someone might read his diary.
When Mario and his Partners came to his room (Cabin 4) he was hidden. Mario had to hide with Vivian for a couple of seconds to make him appear. When he did, he asked Mario to find his diary. Ghost T. warned Mario not to read it.
When Mario finds the diary, he has an option to read it - if he reads it he will get a Game Over (He will have the option to save and read it before returning it).
If Mario agrees to read the Ragged Diary, he will read the following:
Month X Day X Cloudy
After Mario read this, Ghost T. appears, and yells at Mario and co. for not listening to him, followed by giving him an automatic Game Over.