I watched season 1 and 2 on Netflix but there's an OVA which is important or can I continue watching after season 2 directly at book of Circus?
https://www.fernsehserien.de/black-butler/episodenguide
Everything is displayed here
https://www.anisearch.de/.../relations the exact order.
the anime was poorly adapted, so I would recommend the manga.
Is Ciel in Book of Circus a devil or does it play in another time?