The MPEG2 Standard is defined in ISO-13818, which is comprised of the following documents (some of them are also published by ITU):
- ISO-13818-2 aka ITU-T H.262
- Conformance testing (read if you want to write a multiplexer or an encoder)
- DSM-CC (Digital Storage Media Command and Control, used to transmit MHP applications and other data
- Advanced Audio Coding (AAC)
For a full list of the MPEG2 standard's documents, see the ISO website.
In addition, given that the MPEG2 Standard is a superset of the MPEG1 Standard, you will find that it will, in places, refer to the older MPEG1 standards documents (especially in the case of audio coding), so you may wish to read ISO-11172 as well.