The 0v would be GND. If you connect an oscilloscope to the 1.8v TTL pins, one of them should have a series of pulses (ASCII data) while the kindle is booting up. You can hold the power button for about 7 seconds to power down, and another 30 seconds to reboot (or select Restart in the settings menu).
With no oscilloscope, you should see the average voltage drop a little (with an analog voltmeter) or fluctuate (with a digital voltmeter) during reboot.
The fluctuating pin is Tx (out of the kindle). The other pin is Rx (into the kindle). Be sure to connect the Tx from the adapter to Rx into the kindle and vice-versa, or they will not communicate.
Also, it is a good idea to use 1.8v TTL. You can convert the voltage levels like this:
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