None of them have been out for even a year yet, most just a few months, but I think it's illustrative of how slight book sales are in general and why the average author earns £4,500 a year from their writing. I'm not sure - considering some of the comments I see on Twitter and various writers blogs I follow - most people realise how difficult it is for most writers to make a living out of writing. There aren't that many Dan Browns around (resists temptation to make obvious remark).