Postmark Values Increasing Stamp Value By 20 Times Catalog?

I've noticed that some postmarks command ridiculously high premiums, increasing the value of some stamps by a factor of 20 or more! I recently saw a stamp on ebay worth $20 go for over $400 in a last second bidding war just because of a rare postmark from a dead post office.

I had no idea about this element of collecting and I would like to know more. Does anyone have any experience in this area, or is there any catalogue or literature that hilights more desirable town cancels, their scarcity factors etc. Since I collect used with town cancels I figured learning more about this would be not only prudent but fun.

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Thx lorddenning!

Great links lorddenning. I haven't joined them as of yet although they are talking about my newest area of collecting I am enjoying, cancels, postmarks and markings. So many things to do.