Its the spin offs that are keeping GRRM from finishing the main book series IMO. I'm sure theres a book coming out soon about the history of westeros which adds more detail to the already rich background of the ice and fire series.. As John said, there are already several short stories out there like the Tales of Dunc and Egg and one about the Targaryen civil war they called The Dance of the Dragons in which the number of Targaryen dragons tumbled drastically.