If it is a problem with a book, it usually means a process is left running that is draining the battery. To test for this, restart the device when you finish reading for the night. But, do not open a book. If the battery drain stops, then it was the book you were reading.
Other than that, make sure there is nothing near where you are leaving the reader that could be affecting the sleep. The cover has a magnet in it and the device has a switch that the magnet affects. If there is another magnet near enough, this can prevent the device from sleeping.
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