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The W3 docs for ActivityPub cover just about everything from a general overview of the system, actors and collections, internationalization, and security considerations.
Pro Backed by W3C
The W3 docs for ActivityPub are made by and endorsed by the W3C. Because of this you can be sure it'll be kept up to date and compatible with W3C standards.
Con Can be intimidating
Being a documentation and not strictly a tutorial, the docs really throw everything right at you. This can be intimidating for people who are new to the system. For beginners it's best used in conjunction with a separate tutorial.