Sorry to be sarcastic, but we are long past the point where this was necessary or even a good idea. There are so many reading apps out there (even for Windows Phone) that developing a branded one is probably not worth the expense. One might argue that they could be the first serious entrant into some untouched markets but that isn't the case here.
But since they are launching an app, let me tell you a little about it. It's only going to be available in Europe, with the initial launch covering France, Germany, Italy, Russia, Spain, and the UK. You can use the app on 4 Lumia models (Lumia 900, 800, 710, and 610) and you should be able to download the app from Nokia's branded Marketplace.
The app includes all the basic features you'd expect, including multiple font sizes, night mode, and more. The announcement avoids mentioning ebooks availability or format support
, but I would guess that this app and service is being supported by txtr. If I'm right (waiting on confirmation) then the ebookstore should have a decent selection.
Update: The ebooks are being provided by OverDrive, not txtr.