More on 'stated in'Edit
If you're using the stated in property to describe a reference, sometimes what you want to reference won't have an item, so you will have to create it.
Bibliographic information in Wikidata uses the FRBR model, which is the same model libraries use to describe items. Don't worry if you're not familiar with FRBR. All you need to know is that you can express your reference as a work (a book, article, publication) or a specific edition of a book, article, publication. You only need to refer to a specific edition if the information your referencing exists only in a particular edition. Otherwise you can use the work as your reference.
You can see a list of properties that correspond to both editions and works on this WikiProject Books page. They are properties like author, title, publisher, date, that are all common pieces of bibliographic information.