It’s not a good book. For me, as a writer, researching science-fiction horror, it was fairly useful but it is not good.
I’ll jump right to the biggest problem with The Reich of the Black Sun – it’s unintentionally pro-Nazi. The thesis of the book is that Nazi scientists when they discarded as “Jewish science” relativity were able to make incredible scientific and technological advances – including anti-gravity and perpetual motion machines that could power long-ranged submarines – even though the Nazis lacked the wealth and freedom of the West (and, particularly, the unbombed United States that benefited from the immigration the largest portion of Jewish scientists fleeing the Nazis). That’s not an obviously racist thing to think, but it as I read the book, it became increasingly anti-Semitic: the only thing holding back science from technologies like anti-gravity and free energy was the pernicious influence of “Jewish science.”
I don’t think this is intentional on Farrell’s part, just the ignorant blindness of most conspiratologists. Like most conspiracy theorists, Farrell is driven by his passions past the point of all reason. There is no evidence of sympathy for Nazi goals in the book, merely an ecstatic gushing about his line of reasoning that puts Nazi scientists on divergent lines of technological development that lead to amazing places. His prior work is a bizarre theory that the Giza pyramid is an alchemical “death star,” and that it was used to destroy a planet that threatened the earth. This guy doesn’t seem to be ideological, just passionate about his untrue beliefs.