Using the Kindle app on the smartphone works really great.
Just don't forget to really download your books into the device, not to rely on the content displayed in the MyLibrary Home.
Had my phone with me on a hike once and got stuck by bad weather for a full day without cell service - no problem, I've all my books with me - just to find out that the only downloaded book available was Black Hawk down - not exactly the perfect book for a cosy rainy day in the tent.