What are common and more in depth resources for world building?
SE is a great place to get questions answered but there are other sites and books that allow for a more in depth and broad look at certain topics that are in the vein of world building but may focus on one topic, or the specific topic can't really be put in the form of a question easily.
I think community wikis for the following are a great place to start:
Stars and star systems
Linguistics Mytholgy, Legend, and Religion