Hi,
My worldwide interest is "pre-1950", and I managed to get a like new set (
ebay)of Scott's 1955 international stamp catalogs. These are a major help and I've used them quite often.
I also have the latest version of Linn's Stamp Identifier, which has been a major aid to me.
Man, I wish I had these when I was stumbling around the hobby back in the 1960s..................