Hi to all...here are some technical notes for you then. The memory stick is more power hungry than the SD Card, and of course, the internal memory.
Thus, it will need charging more often. The only way this can do damage to your battery is in the charge/discharge cycle...so logically, books should be in the internal memory.
I have not yet experienced any freeze-up on my PRS505. The only reason I keep stuff on a memory stick is the easier way I can access them. That's all. It would have been easier to just see the Sony memory as a usb drive, but knowing sony, that is just a dream.
The best format for a book is RTF. Just plonk your text into word and save it as rtf with a size of 14. Then you can zoom to the level required.