Grey literature databases identify resources that are key to research and scholarship. Many are open resources that go through a quality control process before they are published.
Definitions are scholarly dictionaries, encyclopedias and handbooks that help define terms and provide additional context. Routledge provides an amazing number of handbooks on sport policy, ethnicity, politics...
A selective list includes:
Dictionary of Sports Studies (2010)
Encyclopedia of Sports Management and Marketing (2011)
The Business and Culture of Sports: Society, Politics, Economy & Environment (2019, 4 volumes with chapters on topical reviews, awesome!)
Oxford Handbook of Sports Economics (volume 1, volume 2, 2012)
Research Handbook on Sports and Society (2021)
Routledge Handbook of Theory in Sport Management (2016)
Sage Handbook of Sport Management (2017)
Sage Handbook of Sports Economics (2019)
Theses and Dissertations are important unpublished resources granted and retained by universities as capstone projects, these, and dissertations.
Patents, standards and trademarks are intellectual property resources of potential interest.
Open Education Resources (OERs) are digital ebooks and learning objects that can be used as open textbooks on a wide variety of general and disciplinary subjects.