Hi abby,
I'm also a huge fan of Calibre Companion. There is no better Android ebook library app if you're already using Calibre for your master library on your PC or Mac.
A few things you might like to know...
- There is a Calibre Companion (CC) sub-forum here at MobileRead and the Developer, chaley, is very active in that forum. He is also a major contributor to the main Calibre program and seems to have the patience of a saint (most of the time) when answering queries from the rest of us

- The demo version of CC is, as far as I know, fully functional but limited to only 20 books, so you can try for free to see whether you'll like it.
- CC is not an app which will do the actual reading. You will need to install one (or more) Android reading apps of which there are many. When you want to open a book in CC it will ask you which of your installed reading apps you want to use. As you're a Nook user I'm assuming your books are epubs in which case these are just some of the popular epub reader apps which CC integrates nicely with: Mantano, Pocketbook (as per your screencap), Moon+, Coolreader, UBReader, FBReader, Gitden. Some have lots of config options some are simpler. Have fun trying a few different ones.