A depiction of Santa Claus by American illustrator Norman Rockwell heads up the lots in Christie's seasonal auction of American paintings, drawings and sculpture. 'Extra Good Boys and Girls: A Cover for The Saturday Evening Post' is one of many Christmas front covers that Rockwell created for The Saturday Evening Post from 1916 to 1956. Rockwell's numerous images of Santa Claus have even been credited with the secular, consumerist Christmas that America came to embrace. Previously not considered valuable art, illustrations have become big business, says Eric Widing, Head of America Paintings for Christie's. "It's a new development in the market for American illustration, of the last five years or so. Where previously the American illustrators tended to sell for less than perhaps their art merited, now they've been recognised as a key element to American painting and they're selling for prices well in excess of a million dollars, sometimes in excess of even ten million dollars. So they've come of age and they are now a highly collectable category," he said. Rockwell's Santa is not the only highlight at the Christie's sale. The auction boasts two works by Georgia O'Keeffe. 'Trees at Glorieta, New Mexico' is estimated to fetch a hammer price of between three and five million U.S. dollars Also on offer are works by Edward Hopper, Childe Hassam and Andrew Wyeth. The autumn sale boats the most valuable collection of American art that Christie's has ever offered in an auction. "Despite the fact that we have the richest sale in American painting at Christie's that we've ever had, these things are still incredibly difficult to come by, so it's a happenstance that has brought most of our collectors from all over the country to New York this week to see this show," says Widing. The sale's Western Collection includes several paintings and sculptures by Frederic Remington who depicted the expansion by immigrant Europeans into the American west, often using Native Americans as his artistic subjects. The auction takes place on Thursday (November 29) in New York City.