"Is there an AIRSTREAM FOR DUMMIES book?"
No, but this is their website.
"there's a constant humming in the back where the bathroom is...is that normal?"
Yes. That would be your 12 volt
converter. Many of your systems use 12 volt
"Is it ok to have it hooked up to an RV extension cord if it's raining outside?"
Campground outlets usually have covers. I take it you are plugged into a house outlet? If so, be aware that it is safe for no more than 15 amps (the rating on the electrical connectors). And if it is exposed to the elements, then no, I would not use it when raining.
"One more dumb question...I have a 1973 - 25 ft LandYacht...What is that? A Sovereign
, a Safari, or just a LandYacht?"
All Airstreams of that vintage are "Landyachts". The '73 25' was