I picked up this book, by Miranda Green, because of her qualifications in archaeology, listed on the back, and I was not disappointed. In fact, it was first published in paperback by the British Museum Press, in 1997. This is a look at the archaeological evidence for the Iron Age and Roman era goddesses and a look at what their worship reveals about Celtic society and the position of women within that society at the time. It also delves into the myths that have survived, from Ireland and Wales principally.
It's not fluffy, New Age-y stuff - but it's not overly dry and scholarly either. It's aimed at the general public as well as students of the subject, so there's just enough detail along with a good bibliography for those who want to do further study.
It contains some photos I've never seen before, and covers Gaul as well as Britain, touching on the sexual roles of various goddesses, and their links with the land as goddesses of sovreignty, goddesses of war, and more. The last chapter follows the transformation of some goddesses (like Brigit) into Christian saints.
I highly recommend it.