It's short and very well-written. It's the most valuable read on SQL I've ever seen. Usually I avoid reading books online because it's distracting but I couldn't switch to anything else until I finished it.

It's here use-the-index-luke.com/sql/table-of-contents

Hard to say what was the most useful in this book. It's dense with advice. Maybe the most important thing for backend developers is "Do NOT use OFFSET for pagination".

I wish there was a book about other index types (this one explains B-tree indexes in detail).