Celts

Asterix and Obelix by Goscinny and Uderzo. OK so this is a bit silly but it is fun and it can point up some of the major differences.
Eagle of the Ninth by Rosemary Sutcliff Although this story is told from the point of view of a Roman it is precisely located in England and Scotland and does show the original cultre when Marcus heads North.

Goumbi by Gemma Sales A picture book set somewhere in Europe and it doesn't include the Romans.
Hengest’s tale by Jill Paton Walsh I have not read this so I am not sure what it is about. Can anyone enlighten me?
High deeds of Finn MacCool by Rosemary Sutcliff Set in Ireland this is a retelling of an Irish classic.
Hound of Ulster by Malachy Doyle A beginner reader that tells the story of Cuchalainn, an Irish hero.
Legions of the eagle by Henry Treece
Song for a dark Queen by Rosemary Sutcliff A story about Boadicca
The Sailor who captured the sea: a story of the book of Kells by Deborah Nourse Lattimore