A good place to begin is the article in the December 2002 Survey of Current Business (SCB) article entitled “Benchmark Input-Output Accounts of the United States, 1997” which is available online at http://www.bea.gov/scb/pdf/2002/12December/1202I-OAccounts2.pdf.  This article also includes information about classification, valuation, and the treatment of secondary products in the Benchmark I-O tables.  Subsequent changes in classification and definition can be found in the October 2007 SCB article “U.S. Benchmark Input-Output Accounts, 2002,” at http://www.bea.gov/scb/pdf/2007/10%20October/1007_benchmark_io.pdf.  For a more general, more technical description of the inner-workings of I-O tables, any good I-O textbook will be very helpful.  We recommend Miller and Blair, Input-Output Analysis: Foundations and Extensions (1985).