The Druids by Ronald Hutton and The Apple Branch by Alexei Kondratiev.
John Michael Greer has an excellent article on the Druid revival floating around the internet.
Is there an OBOD group near where you live? I would recommend John Beckett's new book "The Path of Paganism" : https://www.amazon.com/Path-Paganism-Experience-Based-Modern-Practi...
He is a Druid, and the book is excellent, and follows from a Druidic perspective. He sites plenty of other sources in his book as well so that would certainly get you started.
I'm right in the middle of
Blood and Mistletoe; the history of Druids in Great Britain
It's pretty good. The author acknowledges talking with modern Druids in the UK during his research about the book, and gives them props for being reasonable and rational even when history did not agree with their beliefs.