Phony issue.
All Congressmen swear in together in Congress, right hand raised. No books are involved. Then there are "ceremonial" swearings, which, as best as I can tell, are arranged by the individual congressmen. 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.
More perspective on this here.