Currently there are issues with the N1E and the NST/G when it comes to purchasing books.
The N1E will return a network error. This may, or may not, have been fixed as of the time of this post. I am unable to test as I do not currently have a N1E at hand.
The work around for this is to buy, and download, the book via the website and then sideload it to your N1E.
I do not think the N1E will let you refresh the library to download the book without giving your the same network error.
The NST/G has a slightly different issue currently. It will give an error when you try and buy a book via the shop function, stating you need to add a credit card.
The work around for this issue is to buy the book on the website and then refresh your library on your NST/G. The device should be able to download it's just having issues with the purchase.
The Nook Team is aware of this issue and they are working on a fix. No estimate was available as of the last time I was in contact with store support.
I've noticed that an issue was started in the Nook News forum about this, and I replied their with the crux of this message. I wanted to post here as this forum is easier to track than that one.
I do not know why CS is not giving the work arounds I mention above. Both should be perfectly functional alternatives until this issue is resolved.