You are reading it wrong "It has ultimately become one of the best-selling novels ever written, with 50 million copies sold by 2003[59] and over 150 million copies sold by 2007" so 2 years after the films released is what made it popular, its not mentioned how many sold before the films. The 150 million by 2007 is just LOTR books alone, he does have other books that would of become popular also after the films.