Maybe it's the Kama Sutra in all its translations. The Diary of Anne Frank is probably up there, too. Further research is obviously needed. Actually, I was a little surprised by the answer, too. I thought it would be Quotations From Chairman Mao.
Every Chinese person is supposed to have a copy, and there are an awful lot of Chinese people.
But I'm guessing those books are free, so it wouldn't be the best- selling book. English is a relatively young language, true, but it's certainly been around since the invention of the printing press, when book selling kicked into high gear. The Guinness Book of World Records should not be disqualified because it's an annual. Lots of books get updated, including The Diary of Anne Frank. New chapters of the diary were just discovered and will be included in upcoming editions.