If I use a nook app to read my nook books on my iphone does it use up data?

I haven't used the Nook app but similar apps allow you to download the entire book at a time you control (i.e., when you are connected to wi-fi) and then would only use a small amount of data if using features such as syncing farthest page read, etc. If you don't have access to wi-fi then it would definitely need to use your cellular data to download each book.

One option is to go to Settings > Cellular > Use Cellular Data For and see if the app is listed there and turn off cellular data for that app. The same section will show the amount of cellular data used since you last reset the statistics at the bottom of the page.