I'll let the China Daily answer:

...But Naomi Simmons, whose books do not appear on any sales charts, has outsold him [Dan Brown, author of Da Vinci Code] by more than two to one.

Simmons, from Shenley in Hertfordshire, north of London, is the author of "New Standard English," a series of text books for primary school children that has sold 105 million copies in China. But unlike Brown, who has earned US$425 million from royalties for his novels, Simmons took a fixed payment of US$272,000.

Simmons and her co-authors became a phenomenon in China after the Ministry of Education decreed in 2001 that English should be taught in schools. Her book published by Macmillan English and FLTRP, the publishing house of Beijing University has inspired a generation of Chinese schoolchildren to sing songs about how the British use knives and forks rather than chopsticks.