I think you'd need two resistors between GND and TX (of your serial adapter), where in the middle you could grab a lower-voltage TX. Choose high enough resistor values, as a RS232 interface isn't really made for feeding current through ohmic resistors. But my e-tech skills are quite rusty.
There will be more difficulties, however, if your serial adapter doesn't like 1V8 levels on its RX pin (in that direction - coming from Kindle - lowering voltage doesn't make sense).
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