Today, my reader was stuck on the opening book screen.
I was able to boot into recovery and discovered that the partitions 2p8, 2p9 and 2p10 where missing.
Tried to repair the partition table with fdisk, but when I tried to store the new table, fdisk showed a write error. According to dmesg, there have been I/O errors on the internal flash just near to the sectors where the missing partitions start.
It is not the first time that I saw mmcblk2 I/O errors via dmesg. But this time it seems to be the end.
So, to all of you that are stuck on the opening book screen - defective internal flash memory might be a cause
Let's see if Sony will repair it on warranty... I'll try to remove all traces indicating that the reader had been rooted...