Many traditional metrics are limited to journal articles, but there are other ways to demonstrate the impact of your book publication.
WorldCat is essentially a combined library catalog for hundreds of thousands of libraries around the world. Search for your book in WorldCat to see how many participating libraries have added your book to their collections. You might even pull the names of a few institutions you consider meaningful or prestigious that hold your work.
Report that your book has been assigned as required or optional reading in other professors' syllabi--this can show impact on education in your discipline. The Open Syllabus Project will allow you to search for your work and (if found) see the number of syllabi that reference it.
Read more: What do syllabi-based altmetrics actually mean?
Note: This project is not a comprehensive database of all university syllabi.