The following resources are good for definitions and overviews of the objects you are choosing to study.
Unlimited users.
The Oxford Handbooks contain in-depth, high-level articles by scholars at the top of their field. Each Handbook offers thorough introductions to topics and a critical survey of the current state of scholarship, creating an original conception of the field and setting the agenda for new research. Handbook articles review the key issues and cutting-edge debates, as well as providing arguments for how those debates might evolve.

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