The Other Bennet Sister

This is the best Pride and Prejudice spin-off that I have read. The P&P characters are all believably Jane Austen's, but have been developed in interesting directions, and influenced credibly by later events. The story starts before Pride and Prejudice, going on to show the events of that story from Mary's very different point of view. The bulk of this story takes place a couple of years after P&P however, when Mary is struggling to find her place. I found this mostly pretty convincing as well. If I have a criticism it is that there was eventually a bit too much romantic angst, going on for too long for my taste. But degrees of angst are a personal thing, and this may well be right up your street. If you like P&P spin-offs I highly recommend it, especially the audiobook version, read excellently by Kristen Atherton.