(I wasn't quite sure where to put this, but since it's a complete interface skin and relies on the new celtic-type UI, I thought this was the best place for it.)
This is a skin only - it will apply the new look to all of your interface windows. It's fully compatible with any other mod you'd like to plug in to it (unless, of course, that mod comes with its own skin).
To install any of the books or skin sets in this series, all you have to do is unzip the files to a custom UI folder. There is no additional XML needed. Once unzipped, select your custom UI from the Options menu within the game.