I assume that people interested in druids will also want to check the Celtic Cultural History book list, since many of the sources there place the druids within the wider discussion of Celtic cultural history.
Ellis, Peter B. The Druids. Wm B Eerdmans Pub Co, 1995. ISBN 0802837980. This is a popular discussion of the historical and mythological history of the druids, best read next to Piggott’s The Druids. Ellis isn’t as careful about his assertions of fact as I would prefer him to be, and at times he is exceedingly inventive. Don’t take him too seriously unless you find supporting evidence in the work of Celtic scholars (Ellis describes himself as a historian).
Amazon catalog page for The Druids
Amazon UK catalog page for The Druids
Green, Miranda. The World of the Druids. New York:Thames and Hudson, 1997. ISBN: 050005083X. I was disappointed in this book; it’s at best a surface discussion, with very poorly chosen illustrations. Many of the illustrations are imaginative depictions of druids based on romantic ahistorical ninteenth and early twentieth century ideas about the ancient druids. I’ve posted a longer review.
Kendrick, T. K. The Druids. This was the standard text on druids until it was replaced by Stuart Pigott’s book. Though a bit outdated in some respects, it’s still worth reading, particularly if you are interested in the creation of the romantic “noble druid” myths.
Piggott, Stuart. The Druids. Frederick A. Praeger: New York, 1968. Reissued Thames and Hudson, London and New York, 1975. ISBN 0500273634. Piggott was one of the leading British archaeologists of this century, and this book is an excellent introduction to what we know, and speculate, about druids both ancient and more modern.
Amazon catalog page for The Druids
Amazon UK catalog page for The Druids
Wernick, Robert. “What Were the Druids like, and was Lindow Man One?” Smithsonian (March, 1988): 146-66. This is a reasonably easy to read overview of the Lindow II body found in a British bog and a basic discussion of the role of the druid in Irish society. If you are interested in the various bog bodies, many of which appear to be of Celtic provenance, and possibly sacrifical victims, take a look at the bog body reading list.