The Lord of the Rings has sold more than 150 million copies over a period of 60 years [0]. The first American edition sold a quarter of a million copies in 1965 alone, and was the New York Times' Paperback Bestsellers list number 1 by the end of the year.
It's not quite up there with the Bible and Koran, but is definitely 'tier 2' if those are 'tier 1'. The films were made because of the massive historical popularity of the books.
[0] https://en.wikipedia.org/wiki/List_of_best-selling_books
But I have not found stats that break down the "150 million over 60 years". It was published on 1954 + 60 years = 2014. The movies are made in 2001. So what is the % of sales for 48 years versus the 14 years after 2001? I am not sure really.
I like LOTR because I like backstories and the whole book is about hearing what's happening behind the mountains. I could never get into Game of Thrones and my girlfriend never made it through LOTR. To each its own.
It's also the basis of the whole high fantasy western culture, including D&D.