The table below shows that 40% of Amazon’s unitsales fall in titles with ranks above 100,000. That is that 40% of all the books sold are not among the 100 000 most sold titles! (You may have heard the number 57% and that is if you update the graph with 2004 sales figures.)

This phenomenon is called 'the long tail'.

"The term refers specifically to the yellow part of the sales chart above, which shows a standard demand curve that could apply to any industry, from entertainment to hard goods. The red part of the curve is the hits, which have dominated our markets and culture for most of the last century. The yellow part is the non-hits, or niches, which is where the new growth is coming from now and in the future." Definition from at thelongtail.

Read more at 'The Long Tail 101' at thelongtail.
The graph above is from a MIT paper on the long tail issue.
To read the original Wired article is found here.

The question is; how do you reach your tail?