Hi,
When I was preparing for the NFA tests I used "How to Pass the Firefighter Selection Process" by Mike Bryon. It gives general advice on how to do well throughout the whole process. Most of the book is made up of example questions for the NFA tests. Some sections will be useful, some won't. I also downloaded example NFA tests from Fire service websites including the one I was applying to.
They gave me an idea as to the type of questions they ask. My 2 bits of advice: 1) I suggest that not only do you practice answering them but you try timing yourself cause they are quite strict on the day and will stop you when the time is up (Think back to school exams). 2) The maths is simple, just make sure you read the question carefully on the day.
Hope that helps. Good luck.