While on holiday DD (11) ran out of reading material and read 'Murder on the Orient Express' which I seem to recall was pretty innocuous. I'm sure I was about this age when I read lots of AC, but that was in the old days when there was far less choice (when the mobile library turned up every other week). She enjoyed it, probably because it was a 'grown up book' and a mystery and would try another.
What I'm looking for is an idea of which other ones would be good to start with and more suitable for an 11 yr old. Any thoughts?