In many disciplines in the arts, 
humanities and social sciences, books are a very significant medium for 
publication of new research. Databases which only count citations in the
 journal literature are under-reporting the impact of publications in 
these disciplines.
Web of Science added the Book Citation Index
 in 2012. Content is comprised of scholarly books, both Series and 
Non-series that present fully referenced articles of original research, 
or reviews of the literature. It may yield some useful book-metrics for some disciplines.
It should be possible to check for 
citations to a particular publication in any database which includes the
 full text of books (in other words, any database of electronic books). 
The largest such database is Google Books, but it is far from comprehensive.