"As far as I'm concerned, they can swear on a cookbook in these ceremonies.

Prager is creating a flap where there should be none, and most other conservative writers I've seen agree that this is a phony issue. Prager is almost alone in his outrage, and you will note that most articles on this subject trace back to him."

Prager does seem to be alone on this issue and he thinks it's an important issue. I don't think Prager would care much if Keith Ellison did his ceremony with a cookbook or copy of Playboy.
But apparently it has been a tradition to use a bible- a tradition started by George Washington. And there is value to continuing tradition. But more important than this, is that Keith Ellison announced his intention to use the Koran instead of the Bible. His intention was to make a flap or at least make symbolic statement. Which *could be* seen as a Muslim congressman rejecting Bible [even though it is considered as a holy work by Muslims [or Mormons, or practically any major religion on this planet].
Prager would prefer that Keith Ellison bring his Koran AND a Bible, because he thinks that would send message that would be more helpful.