I am indebted to Brent Sampson, President & CEO of Outskirts Press, for the following article found on the net, extraordinarily detailed and full of disclaimers, but mighty interesting for all that... on how the Amazon rankings work.
It is quite addictive, checking in to see whether the book has sold a copy... and picking other books to see where they rank in comparison. Quite a timewaster!
So if you have a book on Amazon, check out what the rankings data means HERE
I also found a helpful graph, GRAPH HERE which shows weekly average sales against rankings.
On this basis, Bubble is selling between two and ten copies a week from Amazon. I am not thinking of retiring on the proceeds!