Vorbis dev - Aug 2005 - Reg. Vorbis codebooks + additional references.

Thanks in advance for any help.

I am a bit confused with the number of codebooks that are used in 
vorbis. Why are multiple codebooks necessary ?(for e.g. in one music 
file, I found around 42 codebooks).

Also are there any published papers that discuss the vorbis encoder in 
detail. I searched IEEE and google, but could not find any (possibly 
published under some other name).
Vorbis -I spec document does give a lot of detail, but I am looking for 
a more generic overivew.

Shank