I guess it could have been the Koran, but it looked like a Bible. It was hardback book sized and had those "shiny" looking pages because the light reflected off the top of the pages and there was a tassel hanging out like a bookmark.
I don't know if it was a prop, to use during the swearing in ceremony or if he regulary carries it around. I didn't notice any of the other Senators carrying Bibles, though.
I don't have anything against someone that carries, reads, prays with a Bible---I just don't like people that use them as props to make them look a "certain" way. I think of Jesse Jackson everytime, because he is one of the most un-reverendlike "reverend" I have seen.