Another book has just arrived in my email looks exciting but long (and a bit expensive at £30)
Blood and Mistletoe: The History of the Druids in Britain By Ronald Hutton:
"As Ronald Hutton's Blood and Mistletoe makes clear, we like the idea of the Druids so much that we’ve made up almost everything we know about them," says Noel Malcolm.
It will obviously be a very sensible book about Druids, but Ronald Hutton is the expert on the subject. ;)