Readers of B&N's forum could have known that you can use the browser to read sideloaded *.html:
(see
http://bookclubs.barnesandnoble.com/...ht/true#M13619)
I tried a plain text file and it worked though all "umlauts" are corrupted. (Softrooted 1.4 nook here)
Sideload a file to a location on to your nook or external sdcard, open the browser, "Go to" -> "Webpage" and give the address as
file:///sdcard/folder/filename.txt - for external sdcard
file:///system/media/sdcard/folder[e.g.my%20documents]/filename.txt - for the nook.
If you plan to use your nook for sideloaded files I suggest you use "calibre" (see forum here
https://www.mobileread.com/forums/forumdisplay.php?f=166) and let the software transform the files to *.epub, a format which nook can do best.