There are a couple of very good options.
You could buy a Bentley manual for the Mk4 Golf, this would help with maybe 90% of the S3 stuff as the manual covers the Golf 4 motion as well which is a similar suspension layout. The standard GTI of course shares most of the parts and structure of the S3 also.
For an even better reference (but no so easily referred to) would be to get yourself a copy of ELSA, which is THE Audi workshop manual installed at the dealerships, combine this with a copy of ETKA and VAG-COM and you have a viable alternative to the dealers system except the 5051 diagnostic machine, which obviously is a fair bit better than Vag-Com.
