None of the Haynes manuals cover the quattro versions in B3/B4 or even A4 B5 versions. But the B3 manual is OK for all 4 and 5 cylinder engines and front suspension info which is common to front and 4 wheel drive cars. The haynes A4 book I have only covers the 4 cylinder engines. Perhaps one of the Audi 100 or A6 manuals may cover the 2.6 and 2.8 12v V6 engines I don;t know.
I have a Bentley manual for my car B3 20v quattro, its basically a copy of the Audi workshop manual and covers the saloon and coupe models in 4 and 5 cylinder guise, the Americans apparenty demand that car manufactures release the official details in the USA.
Have you tried 12v.org site for info on the V6 engine, quite a good site -link
http://www.12v.org/