When I was considering taking the NREMT exam (due to possibly moving out of state), I got Mosby's EMT-B Certification Preparation and Review. I'm sure there are other ones out there, but the description on Amazon specifically mentioned the national test and I've been happy with a few other Mosby texts I've had.
Since I never ended up taking the NREMT exam, I can't really say whether it was a good prep text or not, but in reviewing it, it looks like it would be.