Please rate the book
The reason database indexes are still not understood by developers is the depth of the existing content. Whether it is books, articles or videos - they all have their own problems that limit the ability to understand database indexing.
Blog articles and Youtube videos explain indexes by short examples on the most trivial problems (e.g. a missing single-column index) and won't go in-depth any further. But the minimal knowledge learned from these examples cannot be applied to your more complex queries.
However, published books are written for database administrators (DBA) and teach everything from backup approaches over complex database internals to obscure tuning settings that make it hard for developers to get actionable advice. Most likely, you have already stopped reading at least one of these books with 500+ pages because they include too many topics you are not interested in. In the end, database indexes were only covered on a few pages and are not sufficiently explained because the target audience of these books is interested in other topics.
Who is this book for?
You are a developer who can write SQL queries but struggle with creating the correct indexes to make them fast. Neither do you have the time or interest to learn this topic in weeks - a couple evenings on the couch should be enough.
You also just want to learn the practical knowledge necessary to complete your task and move on to other things. The content should avoid any technical complex database implementation details - you want the core information quickly, concisely and understandably.