The Celts lived in Northern Europe and didn't have a written language until they had increased contact with other cultures. Therefore, Celtic history from the Celtic point of view is sparse. Luckily history books covering the lands of Germany and Brittan often contain sections dedicated to the Celts.
The initial chapters of Our Island Story often refer to the Celts as do The History of Germany by HE Marshall. Both books are free on the internet.
Celtic mythology is filled with leprechauns, fairies, gobblins and luck. Many myths have been written down and the following two books are great places to start.
Tales from Celtic Lands by Caitlin Matthews - This book was great for my third grader. It had beautiful illustrations. Many of the stories involved a peasent or a prince.
Celtic Fairy Tales by Joseph Jacobs - This book is free on Librivox and also free on the Baldwin Project. The stories in this book were a bit longer and better for my fifth grader. My daughter liked the Talking Egg Shell fairy tale best. It was about a troll who switched human kids with gobblin kids.
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