I have no idea what you're doing (and I'm not going to bother reading the link) but a [u]voltage divider[/u] (2 resistors) can be used to knock-down the voltage.
The load (the Kindle) is in parallel with the "bottom" resistor and it will reduce the voltage, so you want the load resistance/impedance to be high relative to the voltage divider resistance. If you don't know the Kindle's input impedance choose resistors that sum-up to about 1K... that should be low-enough relative to whatever the Kindle's impedance is.
Of course the voltage divider only works one-way. If the Kindle has to transmit, or if there is handshaking, you'll need active level shifters to bring the voltage up to 5V.