This is not a book for beginners, the author assumes you already know at least the basics about ASP.NET Core and you're looking to, as the book name suggests, customize the default behavior.
One thing that I like particularly is that each chapter is self-contained, by that, I mean that the author explains how to start a new project and implement the particular example from scratch, often, other authors use the same solution across the whole book, and reuse snippets from another chapter, if you're not following along and you want to test just something you might get into a situation where you can't implement the given example due to some part of the solution is missing, anyway, that won't happen if you try to implement any of this book's examples.
Another thing is that the author doesn't assume his way is the best, and not only suggests you look for other ways to implement that solution but also refers the readers to blog posts and other resources so you can learn a different perspective and decide which solution is the best for your case.
The author explains what's the scope for each chapter and tries to give some examples of use-cases where such customization might be used or important, such as, using a 3rd Party Dependency Injection Container, customizing HTTPS with Kestrel, creating custom ActionFilters, TagHelpers, etc...
This book is what I classify as a "cookbook" and definitely one book to have at hand to use in the real world.