You can now pre-order A Knight of the Seven Kingdoms, the first three short stories (The Hedge Knight, The Sworn Sword and The Mystery Knight), which take place about 100 years before A Song of Ice and Fire Book One, A Game of Thrones, begins!
There is no cover art yet, and this collection / anthology will release October 6, 2015. It will be illustrated by Gary Gianni. The Winds of Winter is not scheduled for release this year. Bear in mind that A Dance With Dragons was announced at the end of March, but published in July, way back in 2011.
Anyway, here's the info for A Knight of the Seven Kingdoms, which collects the first three prequel novellas (with more to come in future anthologies, in sets of three per collection)
Taking place nearly a century before the events of A Game of Thrones, A Knight of the Seven Kingdoms compiles the first three official prequel novellas to George R. R. Martin’s ongoing masterwork, A Song of Ice and Fire. These never-before-collected adventures recount an age when the Targaryen line still holds the Iron Throne, and the memory of the last dragon has not yet passed from living consciousness.