The books were made popular by the TV show a decade and a half later.
They were already popular, award-winning books before the TV show. The first book didn't rocket to sales success quickly, as it was in 1996 and it still took time for word to get around. But the second and third books both made the New York Times Bestseller lists for their years. The fourth hit the #1 hardcover fiction spot on that list in its year.
A Game of Thrones (1996) – Locus Award winner, World Fantasy Award and Nebula Award nominee, 1997
A Clash of Kings (1998) – Locus Award winner, Nebula Award nominee, 1999
A Storm of Swords (2000) – Locus Award winner, Hugo Award and Nebula Awards nominee, 2001
A Feast for Crows (2005) – Hugo, Locus, and British Fantasy Awards nominee, 2006
The show, years later, made the books popular again.