The entire Simon Schama History of Britain series is a great read in its own right and I've found the Elizabethan era helpful for setting up how my Queen's Council works. Also how L'Evallier effectively becomes the Fairy Queen's Prime Minister though he is not known by that title in my stories. But then this was true for William, Lord Burleigh, Queen Elizabeth's Chief Minister. I don't why he isn't credited with being a forerunner for the post of PM as we in the UK know it. Robert Walpole is always credited as being the first. Goodness knows why. Burleigh was in office for a significant time. But it his relationship with Queen Elizabeth that has inspired my writing of L'Evallier's relationship with Roxannadrell.
A wonderful little book called Secret People of the Palace has been a great inspiration as to what goes on behind the scenes in the Fairy Queen's Palace and the kind of people who would be employed. General reference books have also been useful for inspiring what kind of flowers, to name just one example, might grow in the royal magical gardens.
So non-fiction, as well as being a great read in its own right, has enormous potential for sparking ideas off in fiction writers and that is always a good thing.