I forgot to mention having a 1955 two volume set of Scotts worldwide catalog. I got this off
ebay, and since my WW collection goes no later than 1955, the books are a great help and go hand in hand with the Linn's Stamp Identifier book. By the way, new editions of that are less than $12 - money well spent.
Gotta say, wish I had access to this stuff when I collected in the late '50s and '60s!