One the one side there's the operating system and the dictionaries which need some memory, too. On the other side, Sony made a separate "memory"-partition of about 20MB for the installer which shows up as a separate external drive when connected to a pc (so we cannot use this one).
I don't know how many books you store on your device, but even with my fully loaded reader, there are still some MB left over every time I reload
I hope this answers your questions.