The Superbook adds a secondary display to your Surface

[Windows Central]

The Superbook is a $99 laptop shell that lets users connect their Android phones and have a Continuum-like experience. The device offers DisplayLink support as well, allowing you to use it as a secondary display for the Surface Pro 4 or any other Windows tablet.
The Superbook features a 11.6-inch 1366 x 768 display, trackpad and keyboard, along with USB-C and microUSB connectors.

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