I did my NAUI Master Diver a while back. I had to simultaneously prepare for the NAUI Master Diver, Rescue Diver, and First Aide, as well as the LA County ADP program I was attending. One note of caution, it is fine to get the books and study them, but you'll only get so much without instruction. And you might get overloaded doing them all at the same time while trying to refresh your skills.
The ADP program focused extremely heavily on dive tables and physics. It is funny, took us about 6 hours do complete the ADP program final (written exam) and about 30-45 minutes to complete each the NAUI Reccue and Master Diver and NAUI First Aide (all of which multiple choice) and most of us scored in the 92-100% range on both (but 68-75 on ADP, go figure).
Reminescing aside, my point is that you can overload yourself easily. The NAUI Dive Master exam is actually not very simple. There are a few key things you might do well to learn to make it easier: dive tables and physics but the exam isn't only those points.
You'll get some equipment questions, such as care of cyliners and regulators, more about valves than I thought relevant, and even more about bouron tube gauges and diaphragm gauges. But it treads much lighter on the exam than the book leads you to believe. Some about the ocean enviornment and physiology, search and recovery and navigation.
In all, most of which was simple enough if you read the book once or twice (the equipment chapter thrice) and really really really understand your physics though I only got about 7 or 8 questions on it but those took me some time to work out. Brain seems fried after studying for all the other exams all at once.
In short, take your time, enjoy your dives, learn your tables and dive physics, a bit about physiology, all of which as it applies to the NAUI Master Diver cirriculum, and do not stress about it. As a suppliment, I used the NoAAA Diving Manual and Navy Diving Manual v5 at the time, v6 is now current) as a suppliment to get finer understanding.
I found the NAUI Rescue significantly more challenging a test because it seems to be much more rote memorization and procedures than something you learn simply by diving with your favorite NAUI Instructor Trainier bud.
And after you're done, if you're anything like me, you'll probly still use your C-Card for airfills and soon discover you've mastered nothing about diving except elementery dive physics and dive table management. Proud as I am of the card, and as aware of my skills and comfort as I am, I feel like flashing the card would misreprent my true mastery of the craft.