I’ve read some of Peter Berresford Ellis’s works on modern Celtic issues. He’s a strong writer and very honest about his position too, which is a good thing.
You should try finding some of the late Jean Markale’s works. Markale was French writer of Breton ancestry whose works are available in English and worth a read. However, I do feel obliged to state that Markale’s books are/were not without controversy and fierce criticism, nonetheless, if you approach them with an open mind and don’t ever take his word for it, but think about his ideas, i.e. the food-for-thought approach, I think they can be very rewarding and insightful. Some of his ideas do seem a bit far out at times, but I think it’s good that they’re there to be considered.