The two sentences in question contradict each other. One says that 50 million were sold "up to 2003," and that another 100m were sold between 2003 and 2007, raising the total to 150m.
The estimate by the publisher is that one third was sold AFTER the movie's release (in 2001), which explicitly infers 2/3rds sold before 2001, which would be 100m.
Either way, either LOTR sold 100m or 50m before the films, and that makes it pretty popular. Drop the issue.
As an aside, the cultural relevance of LOTR was huge in the 60s and 70s. Led Zeppelin, one of the biggest bands in the world, had multiple songs referencing or outright about LOTR.