There is really nothing else for world-building that even matches reading history. Lots and lots and lots of history.
Indeed, I recommend to all aspiring writers to read lots of it without specific research in mind. The purpose is not to supply you with societies you can rip off, or even nifty ideas you can lift and plant in your stories. The purpose is to broaden your horizons. Or, if you prefer, to knock your block off. It can even get you to the point where you realize that historically, your society is the oddball.
And there is no substitute for primary source when you are trying to get out of yourself. Which is to say, works written in, not about, history. Read the letters of the Regency. Read the diaries of the Victorian era. Read ancient Greek hymns (which will help with religion). Read old Chinese novels. . . .
Which will, as it goes, provide you with lots of things to steal. Lovely, quirky, individual things in the oddest of places. Like Dorothy Osbourne writing to her future husband and telling him about the young women watching the herds.
Secondary source is no substitute for it, but then it's no substitute for secondary source, which is writing about historical events. With, one hopes, some perspective. Good secondary source will tell you how its deductions are derived from primary sources you may never have access to. A book about colonial Massachusetts telling how when the legislature passed a rationing law during a war, it banned all sorts of cakes, cookies, and treats like that -- except wedding cakes. A book about ancient Greco-Roman magic talking about the "curse tablets" found in a temple spring notes that while the thefts the goddess was asked to avenge were all small, the handwriting was all very good and never duplicated, from which they deduce that the practice was so accepted that the victim could easily get professional help for his curse by asking just about anyone associated with the temple.
It also gives you full societies to steal for.
Or, of course, rip off entirely. Not just in historicals. Many an SF or fantasy novel is quite professionally interesting after you have read the right history. Lois McMaster Bujold's Ceteganda and Ivan Morris's The World of the Shining Prince for instance -- or possibly other works about court life in ancient Japan. Martha Wells's The Death of the Necromancer and nineteenth century England. I like a good thick world, and I find many of the best ones are societies with the serials rubbed off.
part of .
Updated: Screening against tangents