Being essentially lazy, my favorite reference source is my blog, where I can throw out any random question and have someone much smarter than I provide the answer in the comments. Failing that, there’s always Wikipedia.
But as for books, I relied on the Eddas for NORSE CODE, both the Younger Edda (or Prose Edda) by the 13th Century Icelandic poet, Snorri Sturluson, and the Elder Edda (or Poetic Edda). The Eddas gave me armor against any readers claiming I got the myths wrong, because even these supposedly official versions can’t agree on how things went down back when the gods were tromping about.
Right now I’m writing what I’m calling my “Los Angeles book,” and I find myself often turning to Los Angeles A to Z: An Encycolpedia of the City and County. Scored it in hardcover for $1.50! It’s great for flipping pages and coming up with ideas for settings. You really can’t come across the entry for the Angel Flight funicular and not want to have a scene set there.