Assuming that your books are sideloaded to your N7
You connect your N7 to a USB port on your PC using the same USB cable come from N7. After connecting wait for 30-40 seconds then go to your computer , click on Computer and hopefully you will be able to see your N7.
Click on your N7 and look in the folder that you have your book and copy them to a folder on your PC.
If your books are purchased from Amazon then you can not copy. It is on your Amazon account and you can always sync it later, all your Amazon book will be downloaded to your N7
One alternative that I used before is to delete your N7 device from Amazon account and re-register it,