If the B&N books have digital rights management (DRM) to make them secure from copying etc., then no they won't work on the Sony readers with the DRM intact. You can strip the DRM away, what we call de-DRM a book, if you have a bit of tech skill and locate the correct tools. Once deDRMed, a B&N ePub can be read on a Sony Reader.

You can also deDRM other formats like MOBI and easily convert them to work on a Sony Reader, which opens the gate to a huge number of Amazon Kindle books as well as books from many other sources.

EDIT: I should include my usual comment that I personally see nothing wrong with deDRMing a book for personal use only. In other words you have a Sony reader but the book is only available in Kindle format, or vice versa. However, even that is illegal in some countries. I do have a problem with deDRMing a book so you can distribute it to other people, and that is definitely illegal in most countries.