E-Reader Devices Statistics

Hello,

Does anyone know where can I get information about E-Book Device Statistics? Specifically I'm looking for the data that would show top 5 devices that are most popular in reading ebooks and how they compare with other less popular devices? For example, let's say, what's the (%) of online readers use Kindle Paperwhire, Kindle Fire, Nook, iPad and so on....?

Thanks,