The full spec itself 'DMX512/1990 and AMX192 standards' is available
from USITT for about US$26. See
http://www.culturenet.ca/usitt/publications.avail.html

There are also pirated copies of the spec floating around the net if you
look hard enough. US$26 isn't much though, and you can grab a copy of
'Recommended Practice' for US$8 at the same time, so get the real copy
from USITT

PLASA & USITT publish a book "Recommended Practice for DMX 512" by Adam
Bennette that discusses usage of DMX512. PLASA can be found at
http://www.plasa.org.uk/ and USITT at
http://www.culturenet.ca/usitt/publications.avail.html

Highend Systems have some DMX protocol info for Cyberlights,
Intellabeams & Studio Colors available at
http://info.highend.com/hes/DLDS/dmxprot.html

DMX protocol is discussed occasionally on comp.arch.embedded and
sci.engr.lighting . Also checkout the lighting control developers list
(see the mailing lists FAQ entry