From the Super Mario Wiki, the Mario encyclopedia
Wiggler's Diary is an optional item found in Paper Mario: Sticker Star. It contains five entries, all of which except one are optional. When the Wiggler's segments get separated from Wiggler, Mario must retrieve them throughout World 3. After a segment is subdued, Mario must lead it to specific places for the segment to create a new page on Wiggler's Diary based on the experiences. The diary entries collected can be viewed at Wiggler's Tree House. Once Wiggler's body is reunited, any optional journal entries that the player skipped over can no longer be obtained.