What I've found is that when I'm reading, the buttons seem to work fine. If I stop reading for a couple of minutes, occasionally the buttons don't work but a swipe on the screen does. For the most part, after the swipe, the buttons work normally. This seems to fit in with what @NiLuJe has mentioned about going into standby and input events being lost.
There are also the times that I've been unable to change pages and when I go back to Home or My Books, I get a popup error message that the ebook has DRM.
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