I love novels about nuns and wanted to include one in my list. Frankly, this post was a toss-up between In This House of Brede, Ron Hansen's Mariette in Ecstasy, and Mark Salzman's Lying Awake. The last two are more contemporary and perhaps more exciting from a plot perspective.
But I liked In This House of Brede because, more than being a story of an individual's journey, it's really the story of a community that, while isolated, changes over time in response to the changing world outside its walls. Great characters, too.