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Are we talking about downloaded B&N books or sideloaded epub files that you manually copy?
For downloaded books, there's an option to download automatically to the SD card (Settings - Download & Storage - Download to SD Card). Not all books are eligible for download to the SD card (it's a publisher option), but I find most are.
For sideloaded books, I found it was as easy as creating a top-level directory on the SD card called NOOK (and a subdirectory called Books) and copied my epub files there. The Nook app automatically saw the books as they were copied there.