A book is also an extension of the mind.
The cell phone, etc will be a much bigger extension. However, we still need better ways to input, organize and access the information. Handwritten notes, voice memos, pictures, drawings, unstructured and structured data, etc.
Cortana/Siri with hand tracking ( http://www.youtube.com/watch?v=0QNiZfSsPc0 ) and a HoloLens will allow people to organize everything in creative ways.