A:
Yes, you can either load the microSD card on your computer, or insert the microSD in the Nook and plug the Nook into your computer with the USB connector; it will show up as two drives, one the internal memory on the Nook, the other is your microSD card.
The card must be formatted in FAT16 format, I think it is; you can format it in the Nook itself. The directory layout for books doesn't matter (though I think screen savers might need to be in a specific location, and not totally sure they can even be in the external memory anyway, haven't played around with that).
I also use Adobe Digital Editions to download books from my library and install them on the Nook. Once you get it set up, it's fairly seamless (you download a small file from the library, which opens in Adobe Digital Editions and downloads the actual book; you then drag the book to the Nook icon. You'll need to "authorize" the Nook the first time you transfer, but it's pretty painless. You'll also need a (free) account from Adobe to use Adobe Digital Editions. ADE also will NOT use the microSD card, it will only transfer to the Nook internal memory - but you can move it manually after it's transferred.