The first part of the book is about the history, the theory and the state of the art of hypermedia and hypertext. The author takes the time to revisit how the hypermedia client (browser) uses hypermedia controls anchor and form to consume the content in non-linear way.
I think it makes sense to take the time for that, considering that many web developers have entered the field in the last 10 years and know only Single Page Applications, developed with frameworks like React and think this is normal.
The Rest of the book is about developing a web application, non SPA of course. Firstly in a web 1.0 style to revisit the mechanics of web browsing how it has been before SPAs and still should be for many web applications.
Then Htmx is introduced and very impressively used to make the web app interactive, similar to an SPA, but while keeping the client server interaction in the original spirit of REST with HTML APIs instead of JSON APIs.
Also a mobile app is developed with https://github.com/Instawork/hyperview
I learned a lot from this book and highly recommend reading it.