Well, if you are plugging the memory card back into the tablet (or into a new one) and it isn't reading it, what is happening exactly?
Also, if the tablet is accepting the card, but not showing the data, have you tried leaving the card in the tablet and then connecting the tablet to a computer via USB cord and accessing the SD cards info like you would any folder on the computer? If it is readable, you should see it then.
Lastly, if the info you had on it was associated with a specific app, you may well need that app installed to reaccess the data. In the case of books you paid for, or apps you paid for, you should be able to redownload them from your account (where you got them from).