Kindle provides the author a monthly tally of ebooks sold, broken down by absolute numbers, royalty categories, royalties per title and total royalties, U.S., UK and German sales. It's very informative. Two of my books have been consistent paid genre Kindle bestsellers since late last year. I need to sell between 500 and 1200/month to make it onto at least one genre bestseller list. But, as stated earlier, it's based on proportionality, not absolute sales numbers.
Nook and other ebook retailers provide more basic sales data. Lightning Source provides fairly detailed data for both print and ebook sales.