Sony 4-153-621-13(1) To delete all the notes within a book,

PRS-600.GB.4-153-621-11(1)
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Reading Books
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You can also create notes (excluding markups and handwritten
comments) in eBook Library.
In eBook Library, you can set to merge the notes of the Reader
and eBook Library while synchronizing. When merging the notes,
any inconsistent note for the same word will be replaced with the
latest note.
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When you copy a book from eBook Library to the Reader, the
notes of the Reader will be replaced with the ones of the eBook
Library (or vice versa).
To delete all the notes within a book
In the Option menu of the “Notes” list, tap “Delete Notes.”
Tap “Yes” to delete all the notes in the list.
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