the direct downloaded ebooks are in a hidden partition. I've not heard of a way to view those short of doing something like rooting.
If you are looking for a backup copy, download Nook for PC/Mac on a computer, then log into your account from the app, and download your books to you computer that way.
You can view all the internal partitions without rooting if you boot noogie.img from an sdcard. You'll get the "Root Forever" startup screen, but the device isn't modified in any way at this point, and all the partitions are exposed when you connect to a PC.
See this XDA thread on backing up the NST for details. Alternative instructions for rooting are here, which begin the same. If you follow these through step 7 under After you have written the sdcard, you'll see all the partitions.