Go to this link. Good information here. About halfway down, there's mention of Dick Gilkeson's "Baseball Card Variation Book". The book's a little disorganized, but very thorough. I bought my copy directly from Dick, but I can't seem to find his email address anywhere.
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Link to oldbaseball.com variations page
bobsbbcards SGC Registry Sets
Shawn's Baseball Card Variation Master Checklist
bobsbbcards SGC Registry Sets
Bowman Baseball -1948-1955
Fleer Baseball-1923, 1959-2007
Al