Books deleted every time I close the app!!

Hi! 

I've just recently started using Adobe Digital Editions on my Macbook Air, and the installation has worked fine, I have the latest edition as well. I could also download the e-book I bought and read it, highlight, etc, without any problems. However, I had to force quit the program due to it being unresponsive, and when I started it again the book had completely disappeared from my library (it was the first time I quit the app on my computer). And ever since then, every time I close the app after reading the book and then open it up again, the book has disappeared. This is highly annoying, and prevents me from saving my work in the form of bookmarks, etc. What can I do to prevent this?! I have tried signing in and out, deleting and reinstalling, etc. But nothing has worked. Does anyone have any ideas?