The author has admitted to being "angry" when he wrote it. He had an agenda, he is not the careful, meticulous historian he claims to be. Obviously, you should decide whether or not to read it, just keep these things in mind.
Black Sun (Sol Niger) was one of the best-kept secrets in alchemy, for it was considered a source of great transformational power. Basically a symbol of unconscious energy, it is a paradoxical and "negative" image that has rarely been dealt with in Western culture. Black Sun breaks up the logical life of the soul and represents the "crack" in our psychological armor through which the light of true understanding can get in. Marlan draws on a wide variety of sources. Highly recommended.
Celtic Myth & Magick: Harness the Power of the Gods & Goddesses