HBO announced the Game of Thrones will be picked up for a second season.

The series, based on George R.R. Martin’s A Song of Fire and Ice books, premiered on Sunday night

to good ratings: 2.2 million viewers watched it in its initial airing. That number grew to 4.2 million, as it re-aired twice. The initial number is low, compared to HBO’s last big launch, Boardwalk Empire, which premiered with 4.8 million, but like that show, Thrones was almost immediately picked up for a second season after its premiere aired.

Analysts are saying that Thrones had a weak lead-in, the Mildred Pierce miniseries, which is averaging about 1 million viewers, and the new series doubled that.

HBO executives hope that Thrones will become like True Blood, which debuted with 1.4 million viewers but grew to become a hit for the network.

Each season is supposed to focus on one of the books, so Thrones fans will at least get a chance to see HBO’s take on A Clash of Kings, the second. The series has been announced as seven books long. Martin announced recently that the fifth book, A Dance with Dragons, has been completed and is due to be published July 12, 2011.