I highly recommend the six books listed below. If you only have time to read one or two, I would suggest Kevin Duffy's book as a first read. He has a writing style that is easy and relaxed, yet informative. The book is a general history that covers every aspect of these great people.
Follow that with the Simon James book. This is another general history, but is much more detailed than Duffy.
My third recommendation is Jean Markale's book. This book goes into detail about the myhts and legends and gives geographical evidence upon which the legends may have been founded. It also has a great deal of data concerning the various tribes that made up the Celtic Nation.
Regarding the websites: I have visited many that dealt with the Celts. Some were good. some were excellent and some were not worth the time to click on their URL's. I'm sure there are many more out there that I haven't visited. Some of them may be great; others not so much. I'm sorry I can't visit them all. A few of the better sites I have visited are listed below. Hopefully, they are still active. Sites come and go so quickly. If you encounter a bad link please contact me.
I don't agree with everything in these books or on these sites and I'm sure the authors won't agree with everything I have presented here. The books are worth reading and the sites are worth visiting. Enjoy your reading and form your own opinions.
Bieler, Ludwig; Ireland: Harbinger of the Middle Ages, Harper and Row. 1966
Bonwick, James; Irish Druids and Old
Irish Religions, Dorset Press, 1986
Web Master's note: A word of caution about this book: it was not taken seriously in the Journal of the British Archaeological Association when it was published (see page 201 of the Journal). They even said that the religions Bonwick discussed have nothing to do with the Irish. All that aside, it is still an interesting read.".
Cahill, Thomas; How the Irish Saved Civilization, Anchor Books, 1996
Duffy, Kevin; Who Were The Celts?, Heritage Books Inc., Barnes & Noble edition, 1999
Ellis, Peter Berresford; A Brief History of the Druids, Carroll & Graf Publishers, NY, 1994
James, Simon; The World of the Celts; Thames & Hudson Ltd, 1993
Markale, Jean; The Celts: Uncovering the Myths and Historic Origins of Western Civilization, Inner Traditions International, Rochester, Vermont, 1993
Raftery, Barry; Pagan Celtic Ireland, Thames and Hudson, 1994
Squire, Charles; Celtic Myth and Legend Poetry and Romance, Bell Publishing Co., NY 1979
Ancient Celtic History
The Celts (Keltz) were a diverse group of people whose empire once spanned the European continent.
Celtic monasticism is colored by the peregrinations of the Celtic Saints and, consequently, is of a very different character than western European monasticism.
European Middle Ages: The Celts
...the face of Europe throughout most of the historical period was dominated by a single cultural group, a powerful, culturally diverse group of peoples, the Celts.
History and Warfare: The Celts
The history of the Celtic people goes back many centuries. The Celts transmitted their culture orally, never writing down history or facts.
Hadrian's Wall ran for 73 miles across open country. Why was it built?
Hadrian's Wall, A gallery of images
Hadrian's Wall was a Roman frontier built in the years AD 122-30 by order of the Emperor Hadrian....
Time Line of the Celts: The Celts as a Proto-Historic People
A chronological listing of the various stages of Celtic development.
Transformations of Celtic Mythology in Arthurian Legend
Arthurian legend is the mixture of countless individuals over some 1500 years. The myth may have a basis in fact; it is certainly possible that an historical King Arthur did indeed exist in the sixth century A.D.
The site of photographer Marcus Fisher who spends a great deal of time photographing Ireland, Scotland and Wales.