I've been reading a lot about how couples feel NFP effects their relationship. While learning NFP with my fiance has been great so far, I'm hesitant to say anything more since we're abstaining from sex until we're married.
I started by reading this study of the impact of NFP. There's a definite pro-NFP agenda, but it's still quite interesting.
Then I found this blog about when NFP is difficult (the comments are very interesting).
Lots of food for thought!