@rupor: first of all, thanks for your effort and your root package. I bought an e-book reader and now I have a small tablet.
Great! 
I made a full backup using your recovery package, installed your minimal root and the APPS2SD and also some other apps. Everything is working fine ...
...
except that I cannot boot into recovery mode. I wanted to apply some other hacks (Wifi ad-hoc networks, disable firewall etc.) and tried to login to the device, but the reader is not recognized by my computer as a serial device, neither with Windows nor with Linux. I tried several methods (TestTool, Home-Menu-Button), but neither of them worked.
When "Opening books" appears and the "slider" is at the far end, it stays there for maybe 30 to 40 seconds (as I said "dmesg" does not show the connection of a serial device and also the DeviceManager in Windows does not show anything). After 30-40 seconds the reader reboots again into normal mode.
I could not make much sense out of the translation of the russian page regarding the usage of ebook_msc:
Code:
"ebook_msc name" PRS-T1 "um recovery"
But I would like to stick with Linux anyway ...
Is this problem regarding the recovery mode related to the root package or am I doing something wrong? Is there actually anything like a root-shell for Android?
Thanks and regards,
Joerg