The Illustration Cupboard, a London gallery specialising in such works, says turnover has leapt 90 per cent in three years. Its founder John Huddy described it as an "encouraging, exciting time". The gallery recently sold an E H Shepard illustration for a 1931 version of Kenneth Grahame's The Wind in the Willows for £50,000. Other works by best-selling illustrators – including the Alfie author, Shirley Hughes, and David McKee, who created Mr Benn and Elmer the patchwork elephant – have trebled in value. Luke Heron, director of the art fund StoryBoard Assets, which invests in book illustrations, said that people tend to invest in physical assets at times of economic uncertainty. At some point, he predicts, illustrations will sell for "silly prices".Auction houses are taking note. On Tuesday, Christie's will sell both newer and rare collections, including an illustration for the 1983 edition of Wind in the Willows by Harry Hargreaves, while Sotheby's has more than 20 lots of illustrations later this month, including work by Shepard, Arthur Rackham and Edmund Dulac.Quentin Blake, renowned for his close collaboration with Roald Dahl, admitted there was more interest in illustration, adding that it was an "everyday language" people had rather taken for granted. He claimed there had been a degree of "intellectual snobbery" in the past, with people sometimes ashamed that illustration was done to commission for money.