When it comes to demonstrating the importance of scholarly works like books and book chapters, it's important to keep in mind your departments standards and what your field considers valuable. To help you make this determination, you may want to speak with your mentor/department chair. You can also refer to the Promotion & Tenure guidelines for the Indiana University School of Medicine.
Unlike journals and journal articles, there are no set metrics for books. Most people rely on book reviews or publisher prestige in addition to citation counts to tell their story about books and book chapters.