It's easy to add PDFs to your Zotero library and automatically import their citation info.
First, enable PDF indexing on the Search tab of Zotero's preferences. Zotero will download and install a small plugin.
Next, just drag your PDF files into the Zotero pane.
Right-click the PDFs and choose "Retrieve Metadata for PDFs." Zotero will retrieve their citation data from Google Scholar and turn them into citeable items with PDF attachments.
If Zotero can't find a match on Google Scholar, don't worry -- you can still save the citation from another catalog or article database, then drag the PDF onto the citation to make it an attachment.
While this feature can greatly facilitate importing large existing libraries of PDFs, the imported data, especially where Zotero uses Google Scholar, is of lower quality than importing via Web Translator from most scholarly databases.
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.