If you have access to an old Windows Vista computer, you can download the old Nook for PC application from
B&N Nook for PC download link. I personally find it flaky but usable on Windows 7, although I usually have to log out and log back in to pick up any newly purchased books. The Windows Store Nook app sometimes has problems with that also. I don't have a Mac, and I don't think it works for recent versions of Mac OX, but here's the
Mac Nook App download link.