Slate has posted a link to this fascinating chart created by Reddit user Joel Tone tracking the adaptation of George R.R. Martin's saga from the novels to the TV series, showing exactly which parts of the books were used in the show and in what episodes they appear. As you can see, the timeline follows a mostly linear trajectory through the first two seasons, albeit with some exceptions — note that a subplot with Bran from Clash of Kings was pushed into Season Three, and a couple of other scenes were moved back to the Season One finale.
As the show moves into Season Three, the timeline starts to decohere as the characters go their separate ways and the chronology gets muddier. And since Season Four is supposed to incorporate elements from the third, fourth, and fifth books in the series, one imagines that the lower right corner of the next version of this chart will resemble an explosion in a confetti factory.