APU's are a great device. If you don't understand exactly what it is, it is a generator that provides electricity to run heat and air conditioning. It runs off of the trucks own diesel, and burns much much less fuel then just letting the trucks engine idle to do the same function. Also most good APU's also recharge the trucks batteries, keep the coolant at proper temps, etc.
The most widely used is Probably
TRIPac. They are quite expensive, but for a Solo driver who drives OTR, it will easily pay for itself.