Well, not exactly. There are a number of fast-charge standards that will now kick specific models of "USB" charger up to 20 V. (see USB Power section of
https://en.wikipedia.org/wiki/USB for some of these). This is not a hazard to a normal USB device like an e-book. I think those chargers have a data handshake with higher-power devices, identifying them, before kicking up the voltage.