Book 3 was split into 2 seasons because it's so large. Books 4 and 5 run concurrent in the timeline so you'll see the first half of book 4 and 5 in first year and the last half of 4 and 5 in the second year.
They might even get 3 years out of 4 and 5 if they wanted to extend it. By that time 6 should already be out. He's already said book 6 and 7 is looking at 1500 pages each. So by the time they get to 6 they'll have enough material to make a couple seasons of it. I can't wait to own the bluray set once this series is complete.
GRRM gave the HBO producers the overview of what direction he's going to take so even if they catch up they can go off on their own tangent, with his approval. I've heard they discussed doing a flashback season like what Spartacus did.
