First the O2 Sensor voltages. The upstream O2 sensor produces a voltage that will bounce all over the place as the engine runs. It's normal to see voltages anywhere from around 0.1 to 0.9 Volts. Now the downstream O2 sensor will be more stable and while it might change a bit, it won't change like the upstream O2 sensor. Typical downstream O2 sensor voltages will be in the range of 0.5 to 0.9 volts.
What is more important than O2 Sensor voltages is the Short Term (STFT) and Long Term (LTFT) Fuel Trim values expressed as a %. Negative fuel trim values indicate that the engine is running rich and that fuel is being withheld (hence the negative value). Positive fuel trim values indicate that the engine is running lean and that fuel is being added (hence the positive value). Now comes the really "important" fuel trim value. This is the sum of the STFT and LTFT values.
What you want with the sum of the STFT and LTFT values is a number somewhere in the range of -10% to +10%. As long as the sum of the two fuel trim values is in the +/- 10% range, things with the fuel system are OK.
Now for the recommendation for a decent OBDII code reader/live data scanner. I use an Bluetooth OBDII dongle and the Torque Pro app for Android devices. The Bluetooth OBDII dongle for Android devices costs around $15 from Amazon (here is the link):
Buy Veepeak Mini Bluetooth OBD II Scanner for Android Only, Auto Check Engine Light Code Reader Car Diagnostic Scan Tool at Amazon. Customer reviews and photos may be available to help you make the right purchase decision!
And the Torque Pro app has a one time licensing fee of $5.00. So for under $20 bucks I can read and erase diagnostic codes, monitor various engine sensors, and I can also store the data in an Excel like file for later analysis on a PC. Note, this is all possible once the Torque Pro App has been set up. I can also have different vehicle profiles for different vehicles so that I do not have to add or remove things to be monitored when going from vehicle to vehicle.
If you have an iPhone, the Veepeak Bluetooth dongle for iPhones is a about 5 dollars more:
Unfortunately, the Torque Pro app is only for Android devices so you'll need to look elsewhere for an equivalent iPhone app.