What does it take to stay in the top 100?

I've been monitoring Amazon UK's Crime fiction top 10 for a long time now. This morning I noticed that C.J. Sansom's DISSOLUTION has been in the top 100 for 738 days - that's over 2 years.

I've written about the list a few times now.

The top 100 list also contains books not yet published which I find a rather odd feature, so obviously when Dan Brown's THE LOST SYMBOL which has already racked up 23 days in the top 100 and is not due for publication for another 90 days, will have a head start.

It is an interesting list to watch because it keeps you informed about what is gaining in popularity and perhaps who you should consider reading. My jury is out on Dan Brown although I have to admit I quite liked THE DA VINCI CODE and DIGITAL FORTRESS was ok.

You start to think though, when an author appears in that top 10 list with 2 titles, and Sansom was right up there early last year; A QUIET BELIEF IN ANGELS, R.J. Ellory was on top for a long time last year particularly after he came to the notice of Richard and Judy; and Alexander McCall Smith, at around the time of the filming of the No. 1 Ladies detective Agency, had 3 books in the top 10!

The top 10 list this morning is

1. THE LOST SYMBOL, Dan Brown
2. THE GIRL WITH THE DRAGON TATTOO, Stieg Larsson, rated 4.8 by me
3. ANGELS AND DEMONS, Dan Brown
4. THE SHACK, William P. Young, rated 0 by me -so that's how I came to read it! My review talks about why it should NOT be listed as crime fiction.
5. MIDNIGHT FUGUE, Reginald Hill - I want to read this!
6. GONE TOMORROW, Lee Child
7. ASSEGAI, Wilbur Smith
8. THE SCARECROW, Michael Connolly
9. THE GIRL WHO PLAYED WITH FIRE, Steieg Larsson - on the shelves, waiting its turn
10. GIRL MISSING, Tess Gerritsen
    

If you want to follow the list be RSS use the link is http://www.amazon.co.uk/rss/bestsellers/books/72/ref=pd_ts_rss_link

So what influences people to buy a book from Amazon UK and thus help it on its way into the top 100? You might have your own opinions. I think lists like this tend to be self perpetuating, and of course there are other influences like discounting wars, advertising hype and the like.