Looking at something similar, the Lord of the Rings had around 80-90 million viewers (very rough estimation from the global box office [3]) while the book sold 150 million copies [4], so the books seem to have fared better.
[1] - https://en.wikipedia.org/wiki/List_of_Game_of_Thrones_episod...
[2] - https://en.wikipedia.org/wiki/A_Song_of_Ice_and_Fire#cite_no...
[3] - https://www.boxofficemojo.com/title/tt0120737/?ref_=bo_tt_ta...
[4] - https://www.reuters.com/article/us-arts-tolkien-novel-idUSL1...