We’re in the process of updating the documentation for Zotero 5.0. Some documentation may be outdated in the meantime. Thanks for your understanding.
Find a book on your library catalog website. A Zotero book icon will appear in the location bar at the top of your Firefox window. Click on the icon, and the bibliographic information will appear on the right side of your Zotero pane. You can then add notes, attachments, and tags. You can also edit citation information. If the page has more than one book, clicking the folder icon will give you the opportunity to select which books you want to add.