Quick-n-Dirty TTL voltage conversion:
EDIT: Several people have reported success just using a 3.3v USB TTL adapter to connect to a 1.8v TTL kindle serial port, so level conversion may not be needed, depending on what USB adapter you are using. If you want to play it safe though, you can try the following suggestions.
You can cut the 3.3v or 5v TTL signal down to 1.4v with a resistor (1K to 10K, but lower is better for high data rate) and two silicon diodes in series (TTL in -> resistor -> diode 1 -> diode 2 -> ground, wired so that the diodes are forward-biased and conducting current). Silicon diodes have a 0.7v drop across them, so two give 1.4v, which should be fine for 1.8v TTL logic.
EDIT: A red LED has a voltage drop af about 1.8v when current is flowing through it. You could use a red LED instead of two silicon diodes. Do not use blue or green LEDs, which have a 3.4v voltage drop. Bright blue LEDs have an even higher voltage drop.
A USB serial cable may be tolerant of the 1.4v low input voltage (try it). If you need to boost the output, you can connect the 1.4v output through a reverse-biased diode to the 3.3v or 5v input, with a pull-up resistor on the higher-voltage input. The resistor pulls the high voltage input pin high, while the diode lets the 1.4v TTL logic to pull the pin to ground through the diode.
You can implement both circuits with two resistors and three silicon diodes.
The downside is that this simple method draws extra battery power through the resistors while idle, so it should be disconnected when not used. You can read more about TTL level conversion here:
UPDATE: Here is the level shifter circuit that I used with a 3.3v TTL USB serial adapter, to recover my Kindle Touch using the internal serial port: