"Relatively few books"on the Reader is a relative term. I might read 100 books in a year (probably somewhat less) and the internal memory holds more than that. So I see absolutely no need to use any memory card. (I don't understand why so many people are obsessed with having their entire large collections in the Reader at once. How many books do you actually read in a month, that doing an occasional transfer is such a chore?)
Besides, I do transfers of new checkouts from the NYC e-library every couple of weeks anyway. And, using memory cards eats up the battery faster, so you'll be 'tethered' to the charger, and to the spinning arrows as the Reader chugs along loading those huge collections.