I am newly Board Certified as an APHN as of Oct. 2012 (Option B). Unfortunately, there are not much in terms of study guides available for our specialty. The one on the ASTDN website is a good resource, and hits the main topics covered by the exam. You would just probably need to expand upon the concepts as necessary depending upon your weaknesses in certain areas. I used this as a refresher before I took the exam. From what I can remember, the exam is similar to the study questions offered by ANCC. Between the ASTDN study guide, and the ANCC reference list, you should be well-prepared to sit for the exam.
Honestly, I did not really study very much for it. This was not my intention of course, but life circumstances such as full-time work, full-time doctoral program, and part-time MSN program sort of got in the way of my study plans. However, I had already registered and paid for the exam, so I had to take it. I used the ASTDN book as a refresher for some of the areas I was weakest on the night before. I do have an MPH and have spent the last 10 years working in public health in some capacity. I did not find the exam to be too difficult, just very long. There are 175 questions and you have 3.5 hrs. to test. You usually can eliminate 2 answers and then are stuck with trying to choose between 2 answers that both seem correct. It's the standard process of elimination and choosing the best answer. I used all my time, and received my results afterward at the testing site. I prayed hard that day, and luckily I received a passing score.
You'll be be surprised about how much you really know. Study hard, eat breakfast, and get a lot of sleep. You will do well. I wish you lots of luck, and I look forward to seeing your post when you pass the exam announcing you've joined the ranks of the rest of us APHNs. We need more advanced nurses like you in this great specialty to keep it going.