You may have difficulties opening your purchased contents when you ran out of storage space on your phone. Just clear the storage space of your old books and try again, you may solve many reading or even video playback issues.
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Long press the book cover in My Contents for around two seconds, and enter the selection screen.
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Click the recycle bin at the bottom and click "Clear Storage Space of Books".
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You will return to My Contents after clearing up the storage space.
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Then click on the book cover, to re-download the file.