Celts: The history and Legacy of One of the Oldest Cultures in Europe by Martin J Dougherty is an easy to read introductory book on the Celts. This book provides an informative and interesting overview of the subject matter, but tends to be rather superficial in detail. To a certain extent, this is due to lack of information since the Celts left behind no written material of their own. The contents of the book is also short on archaeological details. On the other hand, there are no irrelevant personal anecdotes. This is a book that conveys information about Celts and that's it.
The author discusses who the Celts were; what their society was like; their customs; their art and religions; technology; and warfare. There is also an extensive overview of Celtic myths and legends. The author also provides a history of the Celts in terms of their expansion and decline in Europe with a look at how Celts in Gaul and the British Isles fared through history. The book includes numerous illustrations and maps. However, the maps were completely illegible (too small) in the e-book version.
I would recommend this book as an introduction to the Celts for someone new to the subject and not interested in extensive details or military histories. The book may also be of interest to younger readers.