...but nope, it's not Winds of Winter. Rather it's a collection called A Knight of the Seven Kingdoms, collecting the "Dunk and Egg" novellas. The contents will be "The Hedge Knight," "The Sworn Sword," and "The Mystery Knight," all previously published. It might also include a new, non-Dunk and Egg story called "The Princess and the Queen," which will appear first in the forthcoming Dangerous Women anthology edited by Martin, due out this December.
I found it here, on the invaluable Locus guide to Selected Forthcoming Books. (If you like to read a lot of SF and fantasy, you should consider a lifetime subscription, or at the very least, visit that site often.) It only lists the UK edition, with a publication date of June, but you can be sure that's when the US edition will be out as well.
So if a GRRM-reading fan tells you "ERMAGHERD! New Game of Thrones book!" just be sure to let them down gently.