I found the temperature conversion stated in the book was incorrect, or maybe just gave F values when it claimed to give C values. Tbh, there are a few errors or typos in the book and I am just up to Chapter 3..not sure if this is due to my starter kit being the Arduino SRL version.
I ended up changing the code to:
// convert the voltage to temperature, then from degrees F to C
float temperature = ((((voltage - 0.5) * 100) - 32) * 5) /9;
You should lookup the datasheet for your specific temperature sensor online - they typically show charts indicating the voltage and corresponding temperature. For the TMP36 (which is what came with my kit), the data-sheet shows that they report 0.75 volts at 25C, and the chart shows a linear relationship between voltage and temperature. So 0.75 - 0.5 will give you 0.25. When you multiply that by 100 you get 25 (which is the temperature in C). If you saw a voltage of 0.80, that would correspond to another 5C above that (since 10 mV per C would be an additional 5C above 25C, which adds up nicely).