> I am pretty sure Tolkien books were way less popular before the movies were made

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