Building on the Fediverse is hard. Here is a list of ActivityPub and other developer resources that have been helpful in developing Emissary.
This is not a canonical or official list by any means, but hopefully this list of bookmarks is valuable to others who are building their own Fediverse apps.

ActivityPub in PixelFed.

Fediverse Enhancement Proposals

An ActivityPub debugging instance? Gotta take a look, probably.

Another unfortunate Fediverse developer shares their woes.

This is the codebase for castling.club, an ActivityPub server with a single
hardcoded 'King' service actor that acts as a chess arbiter.

This is a primer for implementation of ActivityPub.

Гришка учит Активитипабу. Вовремя!

Fediverse Developer Network

Kinda incomplete

Tedu's implementation of HTTP signatures which is used in, for example, Honk. Since it's written in Go, is actually used, and licensed under a free license, I'll probably use it for Betula.

Info on HTTP signatures.

Lessons learned from making apps that play nice with the fediverse.

Quickly design and test ActivityPub objects with mock servers of popular projects like Mastodon and Pixelfed.

Thanks, just in time.

List of links about ActivityPub development. I'll need that.

Also this:

• https://flak.tedunangst.com/post/activity-notes
• https://flak.tedunangst.com/post/AP-networking