The books are much more detailed than the series which leaves out tons of shit. But if you watch the series from the beggining then reading the books isn't necessary. And there are things the series does better (like adding the ned/jaime fight). The only problem you'll run into is if you lets say watch season 1 and then try to read book 2. There will be so many new characters, destinations, etc you'll have trouble keeping up.
One of my favorite scenes in the second book was tyrion fighting with Balon Swann, which was entirely ommited in the series.