How useful is the IEEE green book?

The Green book should provide good guidance for your grounding of such system. However, you should be able to find better references that are far more useful (and less costly). One of my favorites is "The American Electrician's Handbook". Nearly any edition will provide you with nearly all the types of transformer grounding you are likely to encounter. I actually found it better than my "Electrical Engineering Handbook" in this regard as it provided practical schematics and reasons for various grounding methods vs gobblygook and no wiring diagrams.

 

The Green Book explains clearly the basic principles on which the practice of grounding is built.
Familiarity with it makes a lot of the mysteries of the NEC disappear.

Your plant engineers should have a copy in their library.

I feel that every EE curriculum should have a 1 credit hour course on IEEE 142, Green Book.
That so many electrical engineers graduate lacking understanding of "Ground" should be an embarrassment to the profession.
Just look at the questions we get here on PF about it.

That's my opinion.