The one book that I always fall back on over and over if I just need the comfort of re-reading something, or if I get to a moment in my life where I really need something to make me take a long look at my life, I read "The Pull Of The Moon" by Elizabeth Berg. It's about a woman going through a mid life crisis and she writes a note to her husband and sets off on a road trip to find herself. Along the way she meets lots of people and looks back on her life. Everytime I read it, it just has a profound effect on me.
When I need comforting, I always reread the books that I've loved over the years, because I know that they are guaranteed to give me a good feeling. The one that I've loved more than any other is An Unknown Woman by Alice Koller, which I've read many times. I've come to depend on that book like an old friend.