The final two volumes in the book series (authored by George RR Martin) haven't even been released yet and so the ending in the book series ("A Song Of Ice And Fire") is likely to be much different.
Apparently Martin gave HBO some bullet points on how he felt the series would end but as he is still writing the books, it is very likely that the ending will be much different than what was seen on TV.
At this point, never. So it will be written by a different author who will finally finish the volumes.
I’m not sure he’ll even live long enough to finish the books. His legacy looks like its shaping up like a hot mess of disappointment.