Are any of those books heavy on art work? Or perhaps from third parties (that is, not from the B&N shop). The Nook Glow has a very small user partition (the area seen by USB when side loading), about 500MB, which would hold about 250 books of 2MB each.
I just looked at a few books on my system: a 120 page illustrated childrens book (E. Aster Buunnymund) ran over 10MB, while a text only fiction book was 500kB (0.5MB).
Magazines can be very large files too. (Probably newspapers as well.) Archive what you are done with to free up room.