We believe providing you with the tools you need to build UI is just as important as providing guidelines for how, so we've created a few resources to help you out.
For information on upcoming changes to the Editor and how they effect your packages please check out our Transition Guides.
For a guide on writing effective UX Copy you can refer to the UX Copy Guide.
We're also working on additional resources which will be coming soon.
If you're looking for more information, have a bug to report, or need to get in contact with us you can direct all inquiries to HIG@Unity3d.com.