When I got my Sony I went on a quest for a USB charger to reduce the weight of my baggage when traveling.
I bought quite a few different models. In fact, I tried every model a PSP store had on offer! None would charge my e-book reader.
The only thing that worked was going in to the reader via the round power port.
They might have been USB on one end of the line going into the AC mains power point, but the other end had to enter the reader through the round port, not the USB port.
Stitchawl