A Woman of Independent Means
I've always been a sucker for epistolary novels since I read Daddy Long Legs as a teenager. A Woman of Independent Means is a very good one, covering a woman's life from fourth grade at about ten until her death at seventy-nine.
Apparently it's famous, and the copy I have from the library is the "20th anniversary edition of the beloved national bestseller with a new letter to the reader". Well, I never know a bestseller when I smell it, not even Harry Potter when I happened to be in London about the time it first came out.
This is exquisite, though, touching and realistic, with a strong voice that clearly belongs to a strong woman. The protagonist is based partly (I suspect mostly) on the author's grandmother, influenced by the 1970s feminist movement but without the preachiness that often comes from that.
There's a reader's guide at the back, which can also be found at the Penguin Putnam site; it offers a much more adequate summary of the book than I can.