This book is good. It will make you the PostgreSQL queries optimization ninja.
You will gain a deep understanding of the PostgreSQL indexes. Also, you'll learn how to write dynamic SQL queries and pack them into functions for better performance.
Also, the book will be helpful to programmers who use ORM (Django, SQLAlchemy, etc.). It is desirable to see what queries it makes to the database. It may be a good idea to replace some of these automatically generated SQL queries with more performant ones that you write manually.
This book exceeded my expectations. I strongly recommend reading it if you are interested in PostgreSQL.