Documentation on the IP multicast software is included in the distribution on gregorio.stanford.edu (ipmulticast.README). RFC1112 specifies the "Host Extensions for IP Multicasting".
Multicast routing algorithms are described in the paper "Multicast Routing in Internetworks and Extended LANs" by S. Deering, in the Proceedings of the ACM SIGCOMM '88 Conference.
There is an article in the June 1992 ConneXions about the first IETF audiocast from San Diego, and a later version of that article is in the July 1992 ACM SIGCOMM CCR. A reprint of the latter article is available by anonymous FTP from venera.isi.edu in the file pub/ietf-audiocast-article.ps. There is no article yet about later IETF audio/videocasts.