Other than the samples
here, there isn't much information about xaml or json. However, it (should) hopefully be quite straight forward, as it's just an object graph of the Eto classes.. In Xaml, for example, <Button Text="My Button" /> directly translates to the objects, properties, and events that are available.
If you run into any problems or have questions about the functionality, don't hesitate to ask them here and I'll do my best to answer. I tend to work on documentation and samples as people run into issues, so any feedback on that would be very welcome.
Hope this helps!
Curtis.