My personal recommendation would be from my own library- Official Price Guide to Mint Errors by Alan Herbert. Don't let the title fool you, pricing is not the primary function of this book. Alan Herbert delves into the PDS system to help explain and classify errors and die varieties. PDS is short for Planchet, Die, and Strike- the three areas where errors and varieties occur(not to be confused with Philly, Denver, and San Fran
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