To be fully clear, I mostly have been skimming around the book
If you want to know the nitty gritty of how a modem works internally, this book is probably not for you.
But if you want to know what it was like to use a computer to connect with online services (mostly not the Internet!) back in 1994, this book is an incredible time capsule.
It walks people through installing and setting up a modem, discusses the software they would need, and discusses what services they can talk to.
"Advertisements" from 1994 are treasures to read in 2023. It walks through using the major commercial network providers, discusses hobbyist BBSes, and discusses and walks through using the Internet, showing a time that the "Internet" was not as ubiquitous as it is now, and was not as easy for a layperson to use as the other, now mostly extinct alternatives were.
I now have a whole series of “For Dummies" books. They have helped me to understand complex ideas in simple ways. However, “MODEMs for Dummies" is not worth the paper it's written on. It only contains 10 AT symbols and does not describe them very well. Over half of this book are advertisements for various Internet services. It has a general picture of an internal modem and no comparison to external modems. There is not much more to say about this book, because there is not much more about this book!