The classic article, in this context, is "Digital Waveshaping Synthesis" Marc Le Brun JAES 1979 April, vol 27, no 4, p250 Daniel Arfib, at the same time but independently, did similar work -- I don't know a reference. I think DiPoli wrote a good overview -- perhaps in Roads "The Music Machine"? CLM has code both for the table-oriented and polynomial-oriented approaches. There's an example "brighten" instrument in clm.html.