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That is a weird error, that doesn't happen normally. That means there is a problem with one or more of your software apps. We'll need to reset it to reload your software. This will delete everything you have put on the tab, but your books are safe in the cloud. All you need to do after the reset is sign into your account, and you'll be able to access your books.
Turn it off. Hold the volume +, home and power buttons.
When the Samsung logo appears, release the buttons.
Select "wipe data / factory reset" by using Volume buttons to navigate and Power button to confirm.
Choose REBOOT SYSTEM NOW.
Now sign back into your account, and you'll be able to access your books.