I haunted the library system extensively when I researched my WIP five years ago. (I had only dial-up Internet then.) My acts II and III are set in 1490 Brittany.
I read books on French history from 800 to 1700, the evolution of castle construction, and life in the Middle Ages. I read biographies of the Duchess Anne. I scoured travel guides and gleanings on Celtic culture. (Celts in Wales, Scotland, Britain and Brittany appear to be an oral culture. Archaeologists have uncovered only a few caches of gold coin and accessories, menhirs, and legends.)
I bought books on the Goddess Brigid and Druidism--some new age, some professorial.
My local bookstore sourced two Breton-English dictionaries. I joined the American Society for the Preservation of Breton, a tiny group that passionately supports the Breton movement in Brittany to preserve their language and culture. (The French Government bans the language in Breton schools.)
The Park Rapids library had a fabulous book on historical costumes. I researched castles in Brittany and chose Dinan. I studied piracy on the English Channel, the Rance River estuary, the Celtic legend of Isolde and Tristan.
For an earlier WIP, I studied steam-powered Mississippi riverboats and Army forts along the Missouri River in South Dakota. My husband collects centennial celebration books published by tiny towns in MN, ND, and SD. Great old pictures and historical information.