Miranda Aldhouse-Green who is a professor of archeology at Cardiff University wrote the Gods of the Celts. She has done so much research in this field that certainly her books are always read by people who want to learn about the Celts.
This is one of my favorite books on the Celtic Gods so far. I’ve read it more than three times. And even though one of her theories on the cults of sun and sky have been discarded by other scholars after more archeological finds the rest of the book is certainly very much up to date. The book was first written in 1986 and then revised in 2004.
She introduces you to the Celts by putting them in their historical context and then she tells you about the available evidence to what she is writing in her book and she tells you the problem with each of these sources (which is something you will see in every respectable book on the Celts). Then she goes on to discuss all the different gods and goddesses of the Celts. She also talks about animals and animism, and symbols and imagery.
It is a well researched scholarly work, but one that is not boring. It has pictures and the bibliography at the end is a treasure trove for anyone interested in further reading on the subject.