For the most part, Zotero is able to recognize different types of secondary sources like books and articles in your browser and download the relevant information into your library.
The important exceptions for legal research are Westlaw and LexisNexis. For more information about documents on those platforms, see the Westlaw and Lexis tab.
When downloading and saving pdf articles, right click on the PDF in Zotero and select Retrieve Metadata for PDF. If the article's metadata is available, this function will populate the necessary fields for your citation.
You can also manually add a secondary source by starting a new item and selecting from the list of resource types. As well, you can save information from a book by entering in the ISBN, and having Zotero fill in the rest of the information. While this can help save time, always be sure to check the correct information has been entered.
Check out Zotero's Support page for more tips and tutorials.
York University Library's Zotero guide has additional information that is not discipline-specific.