I didn't find the series spoiled having read the first book. I mean, I knew what was coming, but it was still fun watching it unfold. They also added a bunch of stuff into the series which, in my opinion, essentially does in individual scenes what the books do in some 50 pages of revealing bits and bobs about the characters.
The scene between Robert and Cersei for example appears nowhere in the book, but it gives you a good sense of some other stuff that's alluded to in the book but very gradually and in bits and pieces here and there. Instead, they condense it to one very crisp, terrifically acted scene.
So, I'd say if you're curious, read the books. As has been said here, though, read only the first three, and then wait for the 5th to come out before reading 4 and 5 back-to-back. Apparently 4 and 5 tell two different sides of a single story and happen at the same time, with 4 happening in the south, and 5 happening in the north.