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Frank WakefieldBarry, Ted, and Marty are right in suggesting Scot Reader's T206 compilation, as well as Mr. Lipset's great work of 3 volumes, The Encyclopedia of Baseball Cards. Volume 3 deals with twentieth century tobacco cards.
Any collector really wanting to understand old cards would benefit greatly by obtaining and reading these. Although someone wanting attempting to "invest" in old cards wouldn't really need either of these.