Books
Books can be aimed at different audiences. Look for for books that are written by scientists, social scientists, and other scholars.
Reports by Researchers at Universities and Government Agencies
One way to find these is to do a Google search and then limit the search with site:.gov or site:.edu
Theses and Dissertations
Theses are written by students getting a masters degree (though not all masters programs require them), and dissertations are written by students getting a PhD. A lot of times, the content of theses and dissertations will also be published, in modified form, in peer-reviewed journals.
