Generally speaking, its a basic but broad overview of much of the content he covers in his videos and on his website. In some areas it is not as in depth but it is more comprehensive (with the exception of batteries). It weaves together the various concepts and parts of the system. Its a good beginner level introduction and reference reference.
I've got no issue with that, and totally agree its worth mentioning, But you didn't originally mention the part about kindle unlimited (a paid service that not a lot of people have) you just said it was free, hence the confusion.
Anyways, we don't need to beat a dead horse, its a minor point and its been clarified, we are on the same page I think.
Don't use the term free and then we won't object that if you have to pay for it, it is not free.
Doesn't Unlimited Kindle let you download EVERYTHING in the Kindle library on Amazon? That is not the same thing as Kindle making something free to download to everybody (which they do for individual books on a regular basis).