According to this Publisher's Weekly article, John Locke, author of nine mysteries on the Kindle Direct Publishing platform, is the first self-publishing author to join the 1 Million Club on Kindle.
Congratulations to him. How is he celebrating? By writing the eBook How I Sold 1 Million eBooks in 5 Months. Enterprising fellow, eh? I bought the book. At $4.99, I couldn't resist. Anyone savvy enough to sell 1 million Kindle eBooks should have some decent advice for others, and even if I never follow his advice (or already *do* follow it), it's worth learning more about what fellow writers are doing to drive sales.