Look, let's face it. BMWs are getting increasingly complicated. If you really want to break down your engine completely, maybe you shouldn't be working with such an advanced piece of hardware. (The analogy is like a woodworking hobbyist attempting to build an office building.) So it makes sense that the manual doesn't cover the process of rebuilding an M62 engine to the Nth degree. I've had 9 BMWs and 9 Bentley manuals. My only replacement engine came from a crate. What this manual does cover is the practical aspects of maintianing and servicing your own car, including troubleshooting dozens of issues and maintaining many body, electrical, mechanical, engine and transmission parts and functions. For that purpose it is a great manual.
The second volume all about the electrical system and is almost entirely schematics. Every single wire in every year of every version of the E39 is identified. This may seem like overkill, but I can tell you when you are trying to isolate an electrical problem and need to trace one of the hundreds of wires in the harness it absolutely is not. It's a blessing.
Even if you don't work on your car, do you trust your BMW dealer or mechanic? I thought so. My local BMW dealer is full of "BMW recommends that you replace..."garbage. Buy this manual and you can be sure you'll keep him honest. The next time he makes a recommendation that doesn't seem right. Check it out in your book. Chances are the you'll recover more than the cost of the book in your first visit to your mechanic....if you ever go back.