On Afghanistan's Plains: The Story of Britain's Afghan Wars
I've just finished reading this book, and very good it was too.
There's lots of detail about the three wars, and quite a bit of it (especially about the Second and Third Wars) was new to me. I've had to slightly revise my view that we'd fought three wars there and lost them all, but I still believe that we haven't come out of any of them with much credit.
There's a rather odd foreward by General Sir David Richards which focuses more on the current conflict that the subject of the book. However, overall it's a good read, and I would recommend it to anyone.