I think we've all quickly opened a different book before handing our readers to someone to try
filmo's idea is good, but when you swap cards, it will lose the reading positions of the books on the previous card. Probably better is to load books for general consumption in the main memory and the other books onto the SD card. Ejecting the card will still keep the info about the books but not display them in any lists. When the card is put back in, the books will appear.