Owner’s Manual for the vehicle. With a quick reference guide for your convenience. 318i 318is 323i Sedan 323is. This manual is supplemented by a Service and Warranty Information booklet. BMW 318i 36 328i + / $ BMW 318is 323is 36 328is + $ BMW 328i.
I use a combination of the Bentley, Haynes and writeups found on the internet. I've found most individual publications by themselves to be almost worthless and in some cases out right hazardous. I've stumbled across so many inaccuracies printed in the Bentley that could have caused significant damage and cost me a lot of time/money.
It's always a good idea to check other sources and ask people who have actually performed the task before venturing into unfamiliar territory. Pelicanparts.com has a very informative tech article section as does 318ti.org's knowledge base. There's plenty of helpful information and writeups of first hand experiences posted in this forums knowledge base section. All in all, the Bentley is a good book for the beginner trying to tackle small projects. However, if you start working on something and the Bentley doesn't have the answer or looks unfamiliar to your car, it's best to just come back here, search the forum and then ask for help.