Longbourn, Jo Baker. Pride & Prejudice retold from the servants perspective. By 'eck, it's ingenious the way it intertwines with the original, with a wonderful twist which Cannot Be Revealed. I often felt, when ploughing my way through P&P and other classics of the ilk, that there was another story going on 'downstairs', with a whole other cast of characters, which was never told - so this one was a must-read. And it was well worth waiting for.
Granted, Jo Baker is no Austen (but then, who is?), and the tone of "look how hard our lives are compared to them upstairs" is sometimes a bit shrill. But still, it's very entertaining both in itself and in spotting the events in the original. And even if it didn't have The Twist, I'd still be glad it had been written.