Impact metrics are traditionally easier to assess for authors, journals, and individual articles in STEM fields. In fields that publish primarily in book chapters or in books, metrics may be harder to determine, and it is advisable to collect a wide range of numbers to better show your impact:
How many Libraries have your book (use Worldcat)
Has it been listed as a Best Seller or Recommended Reading?
Search library catalogs from around the world. To assess the scholarly impact of your books, search each title in WorldCat and in each relevant record, check the “Libraries Worldwide” field. It will tell you how many Libraries own that item and also provide the names of the Libraries.
Search results often include "Cited by ##" links below the citation, which leads you to other articles available to Google's robots. Full text may not be available via Google, but the library probably has it in a subscription database or print copy for you.