It's difficult to find records for individual book chapters. Try this instead:
Step 1
Find a record for the book in UC Library Search or WorldCat and download the record. Clean up the record in Zotero.
Step 2
Find the record for the book in your Zotero library. Right-click (or two finger click on a Mac) on the record and pick "Duplicate Item". Now, go into your duplicate record and make the following changes:
Step 3
Link up the chapter record to the book record. Click on the "Related" icon to link the two.
If you have PDFs without records you can import them, but be aware that you may have to do some cleanup work.
In Zotero, create a new collection and call it "PDF Import" with the date created.
Click and go into the new collection.
You can drag and drop your PDFs into the collection to add them to your library.
Zotero will examine the PDFs and use whatever metadata it finds to fill in the fields. The more you do in one batch, the longer it will take to process the metadata.
Caution: This works well with PDFs from publisher sites. Other PDFs, especially if they're scanned items, will require more cleanup work.
Always check your new records to make sure they have all the data you need!
If you have cloud storage available where you can map to the drive on your computer, create a folder just for your Zotero attachments and then link to the attachments from Zotero (instead of attaching a copy of the file). This will reduce the overall size of your Zotero library. The primary benefits of doing it this way are that you can backup your library to the Zotero site without paying for more storage, and you can more easily find attachments in the cloud if you don't want to go through Zotero first.