Solid mix-ins, like chocolate chips, dried fruit, and nuts, are the other category where it is more commonly accepted to measure by volume. These things do not typically affect the fundamental structure of the baked good; rather, they are just there to add flavor and texture.
In this book, you will find some master recipes that lay out a variety of options for you to flavor your batter or dough. Some of these ingredients are given as volume measurements, as they all have different densities. You will have the freedom to decide how much you want to add to your baked goods with a few guidelines.