Newbie question

[Tom Reynolds <103322.1043@compuserve.com>]

Hello Quattro members,
     As a person newly interested in Audi's, I would like your help in sorting
out the various U.S. models and wonder if there is a good book available?  I
searched through my "Classic Motorbook" catalogue and didn't really find
anything too applicable.  What I'm wondering is when the model designations
changed and what each model was.  I know the 4000 and 5000 changed in '88 (?) to
the 80/90 and the 100/200, and I think the 200 was turbocharged, had 5
cylinders, etc. but then there's the A6 and A4, not to mention the S4 and S6.
I'd really like to get up to speed on these, but various car price magazines
etc. only have sketchy information.  
     Can anyone out there give me a brief history, or point me to a good book on
the subject?  I'm thinking of buying a car in the near/up to one year from now
future, and it's between a couple of different marques, Audi being one of them.
Thank you all for your help.

Tom Reynolds
Hereford, AZ