The current version of the CD+G spec., as well as the CD+MIDI spec.,
was published by Philips in a System Description titled "Compact Disc
Digital Audio System, Subcode/Control and Display System, Channels
R-W" dated May 1991. In a letter accompanying this book, Philips
said it intended to submit this document as an ammendment to the IEC
908 specification. IEC 908 was the final, international standard
version of the orginal Philips CD-DA specification (the Red Book). I
don't know whether CD+G was ever actually incorporated into IEC 908.
Getting specifications from Philips is ususally expensive. You can
get a copy of IEC 908 from ANSI by calling 212-642-4900. It goes for
$85 bucks. Again, I don't know whether it includes CD+G.