Excerpt from an interview with VP for editorial development and publisher relations, James Testa"
How large will the index be at the time of the launch?
At launch, there will be about 25,000 volumes in the database. Book Citation Index will include publications from major publishers, university presses, and other major book sources. [...]
Right now, we are accepting books in the sciences published from 2005 to the present. In the social sciences and humanities, [we accept] anything with a copyright date of 2003 to the present.Read the full interview here.
The lack of book indexation in WoS has often been cited as leading to an underestimation of an author's impact in his/her field. Google scholar - freely available on the web - has always included citation from books.