Below are some guides to OAuth 2.0 which cover many of the topics needed to understand and implement clients and servers.
OAuth 2.0 Simplified, written by Aaron Parecki, is a guide to OAuth 2.0 focused on writing clients that gives a clear overview of the spec at an introductory level.
- Roles: Applications, APIs and Users
- Creating an App
- Authorization: Obtaining an access token
- Making Authenticated Requests
- Differences from OAuth 1.0
- OAuth 2.0 Clients
- Client Registration
- Redirect URIs
- Access Tokens
- Listing Authorizations
- Token Introspection Endpoint
- The Resource Server
- Creating Documentation
- Differences Between OAuth 1 and 2
Code and Libraries
You can find some excellent books on OAuth 2.0.