X how ever many times on Calibre.
I prefer Kindle to Nook for one reason, Nook encryption is based on your Credit Card number, Kindle is some arbitrary number (based on name of first book bought?)
Calibre can convert from one format to another, so if you want to download a bunch of free books from Project Gutenberg or PG Australia, you can convert them to any format, Kindle or Nook and read them on your device (what ever it might be). (you might need Apprentice Alf's Tools)
If you or your spouse don't like reading from a screen, you can convert to a word (rtf) format and print them out (a little bit of glue, any kind, and a simple clamp can bind the edges so you can read it like a book. I must add that this is NOT legal for most ebooks. A cheap duplexing laser printer and toner refills can keep the cost down to just a few dollars for a couple hundred page book.