I just got a nook simple touch and I want to know if I can save books I have downloaded from barnes and noble to my sd card. Any help would be appreciated.
Yes. But, you have to sideload them. There isn't a way with a stock NST to move them to the SD card. If you haven't already, download the Nook for PC app to your computer (or Nook for Mac - depending on what you are using). Once you have that downloaded and your B&N books downloaded to it, you can connect your NST to the computer and it will bring up a box where you can move books from Nook for PC over. There is also the advantage of having a backup of your eBooks on your computer, in case B&N ever loses the rights to sell any eBooks in your library.
Also, you can download Calibre. It's a free program that allows you to manage your library. There are also various plugins that you can add on that will allow you to manipulate your library to a fairly high degree.
There are a couple of ways, neither of them very obvious.
1) Download the books to your PC using the Nook for PC app, find the folder where they're stored, and "sideload" (copy) the files onto your sdcard. This will result in duplicate copies in your onboard Nook library if you've already downloaded the same books directly to your Nook.
2) Modify the Nook's Android settings so it "thinks" that its onboard storage for B&N books is on the sdcard. Forum member roustabout developed this technique and posted detailed instructions for doing it.
For more background on approaches to this issue, read these threads: