Adam Whitehead of The Wertzone has just published a great post attempting to answer these very questions. It includes a speculative timeline for how the show and the books could be produced with both the last book and last season releasing concurrently.
2013: Season 3 – A Storm of Swords Part 1
2014: Season 4 – A Storm of Swords Part 2/A Feast for Crows/A Dance with Dragons Part 1
2015: Season 5 – A Feast for Crows/A Dance with Dragons Part 2
Late 2015: Possible Winds of Winter release date.
2016: Season 6 – A Feast for Crows/A Dance with Dragons Part 3/The Winds of Winter Part 1
2017: Season 7 – The Winds of Winter Part 2
2018: Season 8 – The Winds of Winter Part 3/A Dream of Spring Part 1
2019: Season 9 – A Dream of Spring Part 2/longest realistic lifespan of the TV series
As you can see, this means that Martin would be required to publish A Dream of Spring by spring 2019 at the latest, and to give the showrunners material from it in the summer of 2018.
So it is possible that the Martin could stay just ahead of the show. But it would require him to get the last two books out in a timely manner. What happens if Martin is not able to deliver the books on time?
If HBO does overtake GRRM, then clearly they are going still be asking his advice, getting him to write (at least) one TV episode per season and following outlines and information he has given them for the later books. The ending we will get on screen will be, at least in some form, similar to the ending Martin envisages for the books. I can also see the showrunners asking Martin to write the last episode of the series (they may do this regardless of if the books are out or not). Some readers, particularly those most frustrated with the long gaps between novels, may even prefer the idea that they will get to find out how the story ends on screen within the next six years or so, rather than waiting potentially longer than that if the series extends to eight volumes, or if the sixth and seventh books take a lot longer than it is hoped to appear.
Obviously the TV series is not covering all of the subplots and character arcs that the novels are, so the books will still be worth reading to find out how those issues are resolved. Of course, purists who want to read the books first and see the TV show later will be left with the quandary of putting the show on hold and somehow avoiding lots of spoilers for years on end until the books come out, which I imagine will be quite difficult to achieve.
Whitehead goes into quite a bit more detail in the full post, including a good rebuttal of the proposal that Thrones take a few years off to let Martin catch up. Be sure to check it out.
Winter Is Coming: A great post by Adam. I agree with most of his conclusions. The timeline he produced looks plausible, but I’m highly skeptical that Martin could get the last two books out that quickly. I’m also wondering if the series might not need three books to wrap up, which would really throw a wrench into things. If I were a betting man, I would wager that the show will pass Martin and we will see the end of this series on our TV screens before reading it on the page.