There are a variety of books available with several people mentioning some helpful ones, you might also looked at a book by Bahman Farzad "The Confused Photographer's Guide to Photograhic Exposure and the Simplified Zone System". Several of my students have found this book helpful with its diagrams to clear up the "fog" that can arise from reading about this method. It is more difficult to read than to do.

As a rule of thumb i suggest to my students that they shoot a roll at half the ISO and the other half at the recommended ISO. Then we print an image from each to get a sense of how their equipment is performing with regard to the ISO standards.