The internet is useful for some things though, it'll tell you how to
root your Nook so you can install the Kindle for Android app on it and that will allow you to read Kindle books as well as the epubs you already read on it.
Or as HarryT says, you can use
Calibre to convert the books you buy from Amazon to epub format so your Nook can read them (though if they have DRM you have to remove it first before Calibre can convert - google apprentice alf for details).
The Kobo Touch supports mobi format but not that well and only for DRM free books. So your two options are those listed above.