It's not clear how many titles are in the catalog, but we do know that Xiaomi is using a rather unusual sales model. Rather than simply sell the ebooks, Xiaomi charges readers .03 yuan for every thousand characters read, with the revenue evenly split between Xiaomi and the content provider. That is far closer to a Spotify model than a typical ebookstore, and at this point there's no way to tell that it will generate enough revenue for the authors.
Xiaomi is not a major player outside of China, but when it comes to that market they present a serious threat to Amazon. This 3 year old company now has a valuation of over $10 billion and recently snagged Google’s VP for Android, Hugo Barra, who was likely hired to oversee Xiaomi's international expansion.
In China Xiaomi is known for selling high end smartphones with specs comparable to Samsung's Galaxy smartphones but only charging about half the price. Like Amazon, Xiaomi makes their profit not from hardware but when users buy apps and other content.
Back in September 2013 Xiaomi boasted of selling 7 million smartphones in the first half of 2013. They generated revenues of over $2 billion from content and hardware sales in that period, or about what Xiaomi reported earning for all of 2012.
I'm keeping a list of potential movers and shakers in the international ebook market, and Xiaomi is going on it. This is a company to watch.