Save a copy of the project and then delete the Web Layout from the copy. If there is more than one Print Layout, delete one of the Print Layouts from the project and then add the project to the Book.
The Books feature in QuarkXPress® helps you manage multiple-chapter publications. Multiple users can open books and edit chapters across a network, but only one user can edit a chapter at any given time. Books can only contain QuarkXPress projects that have a single Print Layout. Projects that contain multiple Print Layouts or projects that contain a Web Layout cannot be added to a Book.