Barb, I see I still haven't responded to your questions about killers. I'm sorry. I've only just picked myself up from the floor again. The thought of Rod222 and his Travancore always reduces me to fits of hysterics. (I do hope he'll stop it. It alarms the cat when I have these attacks.)
Cancellations, and their values, are a vast field. Rodney gets all hot and bothered over Travancore killers with village post office numbers in the centres. Me? I can take 'em or leave 'em. Many collectors dislike killers because they do their job too thoroughly, and completely deface the stamp. Unless the killer is very rare, or special for some other reason, it's likely to have little or no value at all. If you can find them on cover, and not defacing the stamp
too badly (as in the case of that Jammu & Kashmir/India postcard), they do little harm, but little positive good either.
Where to learn about them? The postmark experts for each country that used them, I'm afraid. I don't know of a single source of all wisdom on the subject. I doubt there is one. I can tell you a little about killers from places like Orchha

and Nawanagar

that you wouldn't want to know about, but nothing about places you
would want to know about. Sorry!