User blog comment:Werthead/Official: producers will not wait for Martin to finish the books/@comment-8331-20130329181145/@comment-26308-20130331142734

To a certain extent, but the best thing from a marketing POV is to get the book out as fast as is humanly possible so GRRM can get cracking on the last one :-) HBO would be all for that as well, I believe.

It's also worth noting that, just as the publishers have no power at all over the TV series (and GRRM has fairly limited powers), HBO has no power and no say at all over the novels or their release dates.