Here’s a book I don’t expect to see on too many lists (and I’m surprised it’s on mine, as I don’t typically like big-property universes like Star Trek): The Final Reflection, John M Ford. It’s an ancient book (by Star Trek standards-1984) set in the original ST:TOS universe, but it did Klingon world-building before that was a thing (it’s not canon, as it did this long before Klingons were fleshed out in any real way). Loved the worldbuilding, loved that it made a then 2-dimensional alien into a rich 3-d society.