@Henrycat: Thanks for the info.
Can you try to get information on which memoryslot uses the least power? CF or SD? Then i can get the apropriate memorycard

And if it uses much less power if you use the internal flash? (Then i would copy my current books to the internal flash) I guess USB is only an option for short reading periods.
Can you copy files from USB, CF or SD to the internal flash?