Update 04/15/2019: Game of Thrones author George R. R. Martin has offered further insight into the difference between his planned final books and the HBO TV series.
During an interview with 60 Minutes, he said that the show, created by David Benioff and D. B. Weiss, “would have to run another five seasons” for it to be completely faithful to the story he has lined up for the final books in the A Song of Ice and Fire series, Winds of Winter and A Dream of Spring.
The show will be very close, though, thanks to “several days of story conferences” that took place at Martin’s house in Santa Fe, New Mexico. “I don’t think Dan and Dave’s ending is gonna be that different from my ending because of the conversations we did have,” he said.
Source: IGN.com Update: GRRM Elaborates on How GoT Show Ending Compares to Books