PostgreSQL for Beginners
Luzanov P., Rogov E., Levshin I.
I got this book thanks to my visit to the PostgreSQL meetup. In 150 pages of this book the authors have put the basic principles of this database, information about the specific software and much more. It took me about four days of regular reading to read all the material and I generally liked how clear and accessible the specific definitions and terms are described in each of the chapters.