Also if the card already has books loaded on it, it might be a problem book on the card hanging it up. If all your books are on the card, the reader reads things off the sd card slower than if it is on the device itself. It's why I downgraded my SD card from 8GB down to 1GB or 2GB. They seem to read faster on the device. 16GB may be too large for it.
But I also agree with Shopaholic, try the card in a different device to see if it even works in the first place. (If it does, reformat it on your reader and that might help). Otherwise I would advise you to go with a smaller size card.