Turn kindle paperwhite into secondary display

It is possible, but involves lots of work and is kind of useless.

You wouldn't call the end result a monitor, as such -- the Paperwhite logs into an SSH session, so it pretty much only works for terminals, as the lag between wireless keyboard and the e-ink screen is around 200ms.

An article about this exploit is published by Max Ogden :
Kindleberry Wireless: A Portable Outdoor Hackstation.

I will not reproduce the list of required hardware, but here is what the finished product looks like as a complete Raspberry Pi workstation :

When I first got my Kindle I wondered the same thing :) But as far as I know Amazon controls the all the code on the Kindle so it's unlikely that you could do it via USB. If you could capture your display (e.g. using VNC or something similar) and output it to a secure website you could potentially view it with the Kindle browser. However I find the experience of my Kindle's browser disappointing, especially for content that changes (as your screen output probably would).

Also according to http://cutlerish.tumblr.com/post/1272166726/kindle-browser-doesnt-support-ajax the browser doesn't support AJAX so that would make it hard to have it as a realtime display (you'd probably have to click refresh each time).