The first title about the Celts is so far the only serious book about the Celts and their religion that I know. It was released in a softcover, while from number three on, the series come in a hardcover. J.P. Boosten did a wonderfull job explaining how he got his information, who the Celts actually were, he gives a lengthy explanation about the religion with an historical overview, speaks about the temples, rites and magic, druids, afterlife, folklore, etc., etc. The book is only 240 pages, but extremely informative. I am not familiar with the subject enough to know if in the last 50 years information has been disproved or enlarged, but I am very satisfied with this book, also because it has many wonderfully-looking pictures, which seems to be characteristic to the series.