Basically it reprints two articles for transformer builders:
1. Design and Building Transformers, Technical Service Bureau Bulletin D-111, 1938, second edition.
2. Induction Coils by Charles Underhill, extracted from Standard Handbook For Electrical Engineers, 1922.
Basic understanding of transformer design is a very useful bit of knowledge to have. I used a very rudimentary understanding to create a spot welder by rewiring a microwave oven transformer.
I have also seen people create small power grids using home-built transformers.
If you can design transformers you can take the power you have and transform it to the power you need (within reason).
If you have high voltage at low amperage, and need low voltage at high amperage you can do that or the reverse. It is really just simple math that this book easily explains.
I have this book in my library, and I use it as a reference, when working on circuits. but it really isn’t something I use all of the time, but it is something I think would be invaluable if society ever had to rebuild itself.