Watership Down is published by one of the 'Big 6' publishers — Simon & Schuster — so the publisher sets the price to be the same everywhere. Some booksellers may ask more, but nobody is permitted to ask less than the publisher's Mandatory Retail Price (MRP).
Windrusher is being e-published only at Amazon right now. That was the publisher's choice. You can email publisher(at)oceanpublishing.org and ask them to e-publish at B&N or elsewhere. However, the Amazon e-books don't have DRM — all three say Simultaneous Device Usage: Unlimited — so you could buy them from Amazon and convert them to EPUB using Calibre, then sideload the EPUBs onto your NOOK.
Two questions: 1. How much memory does Callibre need? 2. How do you download Kindle ebooks if you don't have a Kindle?
1. How much memory does Callibre need? 2. How do you download Kindle ebooks if you don't have a Kindle?
1. I don't know. Not much. Check the Calibre site; it might say.
2. Install the free Kindle for PC (or Mac) program from Amazon, and it'll automatically download the e-books. You'll need to know the Amazon "ASIN" identifiers for the e-books, because that's what the file names will be. Those are on the Amazon pages, and in the Amazon URL for the pages, for those e-books, so they're not hard to find.