Would it be safe to treat desk top computers and stereo receiver and speakers etc, in this device? Also, how long does one load take to treat?

Yes, it it safe to treat electronics in a device like this. The components in most electronics will withstand temps higher than 130F. Most commercial electronics have a rated max around 70C (~159F) . I'd say it takes 60-120 minutes, depending on what you put in the device. You get a basic thermometer to track the temp. Put the batteries in the "outside" thermometer on first, then put the batteries in the one with the LCD display. Bury the outside therm as deep as you can among the items in the enclosure. You should see temps on both sides of the display. I got it to treat luggage after trips and thrift store finds. It holds quite a bit, and folds to a small size for storage.

Some library books I get have a thin plastic protective cover; will these covers hold up under ZappBug's temperatures?

I am not sure but i know the heat can be quite high inside. I have measured it at over 145 degrees in different places. It was high enough to melt a medicine bottle one time and melt the glue on the sole of and old pair of shoes another time. So my guess is it could melt the thin plastic protective cover.

how long is the treatment?

You have to wait until you hit the desired temperature. I use two wireless thermometers, each one in a different area. If you have a large amount of clothing and materials inside then it can take 6 hours or more to get to the desired temperature. A typical treatment of 3 or 4 suitcases, some shoes, books, toys etc may take 3-4 hours for both of my thermometers to get up to the right temperature. See below from the Zappa Bug websiteThe higher the temperature, the quicker bed bugs die. At 106 F (41 C) it takes over 1.5 hours to kill bed bugs. At 120 F (49 C), it takes less than one minute to kill bed bugs. We strongly that you follow the industry standard of heating your items to 120F for one hour to ensure that all bed bugs and bed bug eggs are dead. Here is a scientific paper from the University of Florida showing this research:http://entnemdept.ifas.ufl.edu/sepmc/Heat_treatment_paper_PereiraEtAl_09.pdfIn addition, we at ZappBug conducted our own experiment with killing live bed bugs at 120 F in one minute. We used a water bath to ensure proper control of temperature and shot a video of the experiment. You can see the video here: http://www.youtube.com/watch?v=QdNjOmbhTFUThe temperature inside the ZappBug Oven typically ranges from 120 F to 145 F.

How much of a hit does using this give your electric bill?

The heater is rated at 1.5 killowatts (1500W) and the average cost of electricity in the US is $.12kw/hr. So at the recommended/max treatment time of 6hrs they all multiply together to get $1.08 per run.