Worldbuilding books are generally filled with fluff. The absolute best unsung book to understand worldbuilding on an academic, comprehensive level is by JP Wolf, titled "Building Imaginary Worlds: The Theory and History of Subcreation".
Here's a book you'll find interesting https://www.amazon.ca/Building-Imaginary-Worlds-History-Subcreation/dp/0415631203/ref=sr_1_1?keywords=world-building+mark+wolf&qid=1568895198&sr=8-1