This is a known issue.
There are several threads
here about this already.
This problem effects several models, and YES, Sony does know about it.
Sony is aware of the issue, and says they are working on a fix
In the mean time, at the above linked support page Sony suggest the following . . .
Until a permanent resolution becomes available, follow this procedure as a workaround for this issue:
1. If your Reader Digital Book is still on, turn it off.
2. Turn your Reader Digital Book back on. While it is turning on, avoid tapping the screen.
3. After it has started and reached the Home screen, wait approximately 60 seconds before opening the book you would like to read.