I think her mother knew she had something to say but was not "given permission" by those in her life. She knew that it was not only okay but necessary for her to write and share her knowledge, observations, wisdom but did not. She knew because she was making a place for her words, but for whatever reason was not willing or able to put pen to paper.
Her generation, the culture in which she lived may have contributed to her feelings that writing or reflecting was not valued. I feel she did not want Terri to see that she had empty volumes because it would make her sad as it did me. She knew Terri would have tried to help her begin to fill the books but at the end of life the task would be too great.
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