Of course there are also the writings of the early Church fathers, men who in some cases knew the Apostles personally. And the Didache. It's about as well-documented as anything that old can be.
As for Matthias..who by the way we here very little of...it is interesting to note that the disciple's decided they should pick the replacement when Christ already had in mind His own choice....all other disciple's were appointed by Christ so now they determined to jump ahead of God and control the situation by filling the space with someone of their own choosing. But I believe that since all the disciple's saw Christ in the flesh...(Matthias did not) I believe that Christ himself selected Paul and He was the replacement for Judas, because He certainly saw Christ on the road to Damascus....Thus Paul was the replacement for Judas. Matthias rather sinks out of the picture pretty much.