Great topic! Thanks for starting it, Partime! Geo-political changes are part of the fun of stamp collecting. And as it's closely tied to history it hits one of my other interests.
I like to use
The Stamp Atlas, by W, Raife Wellsted, Stuart Rossiter & John Flower. It supplies a little background explaining why the changes happened. My copy was published in 1986 and is way out of date,but it's still useful. Often when I'm looking something up I end up simply browsing.
And thanks a lot, KirkS, for helping me kill even MORE time!

Great link!

Karen