This guide to using the app might help User Guide
Do you mean a book that is online or an actual physical book?
Either way you can use the Citation Assistant in Wikitree Sourcer. This is a dialog that you fill in and it creates a citation using the formatting options that you have set in Sourcer.
You can get to the citation assistant by using the Sourcer menu on any page that Sourcer does not yet support for citations. For example from the Sourcer options page.