There may be times when you need to add an item to your library by hand. If so, use the green "plus" icon and choose the type of item from the dropdown list. A window will open with the appropriate fields to complete for that item type.
Have the book in front of you and want to add it to your Zotero library without having to search for a citation?
If you have a book's ISBN, an online article's DOI or PMID number, just click the magic wand button: "Add item by identifier." Type in the book or article's number, and Zotero will automatically download its information and save it to your library.