Writing is all about reading. At any given time, you should be reading at least five to ten times as much as you are writing. Japanese novelist Motoko Arai is known to read a book every day as a form of pre-writing ritual. She would not start her day’s writing until she has finished a volume. And if you are not writing, you should be reading more than when you are writing.
If you intend to write a story that entails a great deal of world building, you should read a great deal more than the usual fiction writer. Specifically, you should be reading non-fiction. Books on history are the most important sources. Dig up as much details as you can about obscure periods of history like the Capetian Dynasty, the Muromachi Shogunate or the Banten Sultanate. Find out how events unfolded and why.
You also need to read a lot of philosophy and psychology. Works of Freud, Jung, Jaspers and Heidegger as well as tenets of Zen Buddhism and Taoism frequently appear in science fiction and fantasy novels. If you have wise men and wizards, they need to sound wise. C. S. Lewis not only wrote The Chronicles of Narnia and chaired the Mediaeval and Renaissance Literature at Cambridge University, but also wrote deeply controversial works on philosophy, theology and Irish history. So evidently, he had studied a lot.
Scrounge for obscure words from odd places. Characters in Frank Herbert’s Dune fly military vehicles called ornithopters, which is a word for a flying machine with flapping wings described in the journals of Leonardo da Vinci.
You should also learn different languages. J. R. R. Tolkien studied Old English, German, Icelandic and Welsh. Ursula Le Guin studied French and Italian. Anthony Burgess made good use of his knowledge of Russian. They not only provide sources for strange sounding words, but also open doors to a wide array of folklore, historical facts and trivia that is not available in English. One of the best books ever written on the lives of the samurai is an eye witness account of a Jesuit minister available only in Portuguese.
All this makes a book involving heavy duty world building very difficult to write, and not really a recommended project for a young writer. When young Ken Follett proposed an idea for an historical novel about building a cathedral that required elaborate world building, his agent talked him out of it. He finally published The Pillars of the Earth at the age of 40. Frank Herbert was 42 years old when he started writing the Dune series. J. R. R. Tokien was 45 when he published The Hobbit and 62 when he published the first of The Lord of the Rings trilogy. George R. R. Martin was 48 when he published the first of The Song of Ice and Fire books titled A Game of Thrones. J. K. Rowling was a mere youngster when she finished the first Harry Potter book at age 30.
If you have read over a dozen books on how to write fiction, it is quite likely that not one of those books teaches you about world building. There is a good reason for that. World building is one of the most difficult elements of writing fiction. If you have mastered plot, pacing, tension, conflict, point of view, structure, dialogue and style, you are ready to write a book. But if you are trying to write a historical novel, or a science fiction/fantasy that requires a lot of world building, it is a completely different story. It takes a great deal of knowledge and research to build a convincing world. This is why good world building is so rare.
Inspiration can come from very strange places. Tolkien’s Middle Earth was inspired by Nordic and Finnish mythology. Frank Herbert’s desert planet was inspired by Abe Kobo’s The Woman in the Dunes. Martin’s Westeros was inspired by Ivanhoe. It requires a lot of quirky reading habits to come across an inspiration for an original world.
But if you enjoy reading non-fiction, in particular, history, philosophy, psychology, religious studies, linguistics and trivia, you just might be the right person to embark on a massive world building project. Fair warning in advance: It could take a lifetime.