Preparing App Submissions for Apple Watch
developer.apple.com
developer.apple.com
It's very early days with WatchKit. You can understand most of the SDK by just looking at the header file for WKInterfaceController. A Watch App is basically a Today Widget Extension.
The WatchKit SDK can seem almost frustratingly limited at times, but I think that's for the better: I'd wager that the first crop of 3rd party Apple Watch apps are going to be a lot more rock-solid than the first crop of 3rd party iPhone apps, due mostly to the focus imposed on us developers by the constraints of the SDK.
I found the API easy to use, it made for quick development due to the limited scope of what can be done. It seemed like if there isn't an obvious way to do something Apple doesn't want it done.