Seek for best indexing solutions, count who purchase most number of books?

Hello everyone,

I want to count top N book buyers. I have data like this,

  1. For book 1, user 1, user 2 and user 3 purchased;
  2. For book 2, user 2 and user 3 purchased;
  3. For book 3, user 3 and user 4 purchased.

If I want to select top 3 book buyers, they are user 3, user 2 and user 1 (or user 4).

Any thoughts how to index book purchase information and select top N buyers, is highly appreciated.

BTW, I just need to count for top N, no need for sorting inside top N.

thanks in advance,