 Zotero makes it easy to capture info on your screen, but you can also manually add in a source's information. Let's save a book. Select the green circle with the white plus sign. This is add new item. Now I can choose book, and a new blank item will appear. Over in the right side window, I can add all the info. The title, author, publishing information, etc. Any of these fields I can fill, I will. Then simply deselect, and the new item is created.