At the request of my mom, FI and I have been reading "Things I Wish I'd Known Before We Got Married" by Gary Chapman. Of all the books we've read preparing for marriage this is by far our favorite, and in our opinion the best.
Gary Chapman is also the author of "The 5 Love Languages" if you're familiar with that. (which was also a helpful book)
So what was your favorite or most helpful marriage book?