Well, https://matrix.org is a good place to start (but could use a face-lift for non-techies). There's a good list of clients, servers, SDKs, etc at https://matrix.org/docs/projects/try-matrix-now

And of course, there is the Matrix spec itself, but I learned most of what I know idling in matrix channels (#matrix:matrix.org and #synapse:matrix.org) and hosting my own synapse instance.