Unless they've made a deal with one of the publishers since the last time I paid attention, there is no official source book for the NREMT.
There are three main publishers of texts for EMS in the US, AAOS (Which is the one you've got.), Brady and Mosby (Which is actually Jones and Bartlett). There may be one or two others out there, but if so, I've never heard of them.
All three of them teach to the NREMT, and in fact, a bit beyond it, since they cover some things that are in some states scopes, but not in NREMT scope.
Short answer: If you learn the material covered in the AAOS book, you'll know everything you need to know for the NREMT.
Aside: I've used the AAOS twice, and Brady once, never used a Mosby, but I have leafed through one. Although AAOS and Mosby are fine, my money is hands down for the Brady in terms of doing the best job, if you're having problems with the AAOS, I'd suggest trying to pick up a cheap used copy of the Brady and supplementing with it.