You could try the IAAO, they have courses and lots of literature on taxation. I'm sure there are some books on appeal. If your State is anything like Michigan, tax appeal is highly litigated. So most of the topics that would be pertinent would be books about how to be a credible witness, writing reports for litigation, etc. The best books in that area are written by the ASA, but a really important piece of information would be finding the rules of evidence for your State, and learning what passes and what does not. You may also want to look on your State website and look for previous opinions issued by your tax court and see how they have treated issues in the past.
Tax appeal is generally considered some of the most difficult assignmnets in our industry. Just having compentency to appraise a property, does not mean you are competent to appraise for tax appeal. It requires a heavy knowledge of your tax law rules and regulations, definitions, and rules of evidence. It also means that you need to have strong expert witness skills, given most of your time on a tax appeal assignment will be preparing and presenting in court.
So be careful...