There isn’t much support yet for commercial eBooks, but you can buy Kindle eBooks. You can also get eBooks and audiobooks from OverDrive via their app. And Wattpad has expanded their community of readers and writer to Windows Phone. Want to listen to audiobooks? Got you covered. Last but not least, Adobe has released a PDF reader.
There are more apps, and there are more free apps, but this should be enough to get you started.