Problem: I keep seeing this "Bentley" manual mentioned and have no clue what it is or if I need it.
Cause: You're new to the E28.
Solution: The "Bentley" manual is the service manual for the E28 (US-Spec, non ///M cars). It is published by Robert Bentley Publishers ( ISBN: 0-8376-0318-8 ). With the exception of the factory blue binder, it is the service manual for the E28. It's the best $40ish you'll ever spend on your E28 - helpful even if you don't do the wrenching.
Where to get it: As of this writing, amazon.com has the best price: $37.77 with free shipping.