a song of ice and fire - Does each season of Game of Thrones roughly equal a book?

This is how it has broken down so far (more or less):

• Season 1 (2011): A Game of Thrones


• Season 2 (2012): A Clash of Kings


• Season 3 (2013): The first half of A Storm of Swords (aka Steel and Snow)


• Season 4 (2014): The second half of A Storm of Swords (aka Blood and Gold)


• Season 5 (2015): Both A Feast for Crows and A Dance with Dragons

Guesswork from the news (see update):

• Season 6 (2016?): The Winds of Winter (unpublished)


• Season 7 (2017?): A Dream of Spring (unpublished) ?

Seasons 1 and 2 were a book each, but it was changed for Season 3, which became the first half of Storm of Swords, and Season 4 became the second half. The producers then, somewhat surprisingly, managed to squeeze the next two books, A Feast for Crows and A Dance with Dragons, into Season 5, by removing lots of what many people saw as extraneous plot lines.

It's worth noting that there has has been some overlap. Certain events have drifted into different seasons, so the above isn't always strictly true -- just mostly true.

Either way, this means that Season 6 will take most of its story from The Winds of Winter (although there may still be unused things from A Feast for Crows and A Dance with Dragons that appear in it).

Update: It appears George RR Martin has confirmed my suspicions in a recent Vanity Fair article.
George R.R. Martin Has a Detailed Plan For Keeping the Game of Thrones TV Show From Catching Up To Him

Update 2: It's been confirmed that the cast have been signed on for a seventh season, and that the creators have hinted about ending the show in the seventh or eighth season.