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Book C is the name given to the specification of DVD-Audio, a format that provides very high fidelity audio on DVD discs. There are various supported audio formats including PCM (up to 192kHz), Dolby Digital 5.1, MPEG-1 (stereo, audio layers II and III), MPEG-2 (multichannel), DTS and SDDS.
For copy protection, DVD-Audio uses Content Protection for Prerecorded Media (CPPM). To this day (July 2007), no method has been found to remove watermarks from DVD-Audio stream, so creating duplicate discs is not really possible. However, it's copy protection was overcome in 2005, with methods to copy the audio without a lossy digital-to-analogue conversion.
There is also...
Book A (for physical DVD format specification)
Book B (for DVD-Video)
Book D (for DVD-R)
Book E (for DVD-RAM)