In principle, this is a good book with a great high-concept: World War II era espionage with a supernatural twist. Somehow, through magical means, the Nazis are able to locate many of the supply convoys that are aiding the British and destroy them, which leads our heroes to engage in their own supernatural battles across the astral plane to solve the mystery and to bring victory to the Forces of Light with echoes of the real-life military intelligence conundrum of how to crack the enemy's advantage without exposing yourself or giving the enemy time to switch up the rules of engagement before you can actualize on it. Indeed, their first true triumph, tracking the evil to its source, is only the beginning of their struggle as this leads to them suffering physical and metaphysical assault, driving them to a desperate flight towards the enemy to try and gain victory before they are worn down. There are crash landings and desperate escape from mobs and bombs and evil elementals and the summoning of Great Old Ones and corpse mutilations all as stand in for some of the real problems that faced the British at roughly the same time the novel was written and takes place in: espionage, cipher breaking, double agents, and important personal battles in out-of-the-way locales.
In the flesh, though, the book wears several deep flaws that mar it or at least flavor it. Though it is clearly wartime pro-British propaganda - with asides including mocking the effectiveness of the Blitz, praising the English spirit, attacking the French's surrender, chiding the Americans for not being involved (this was prior to Pearl Harbor), casting the entirety of Germany as being implicit in the Evil of Nazis - rarely does it chafe as such and even when it does it is largely forgivable being written in 1941 (one ending line about Anglos being the true Guardians of Light and the only hope for the entire world being a bit OTT). It's theosophic world view likewise is not too terribly overblown on the whole, though given towards long speeches written out like bad Socratic "dialogues" where a main character will essentially information dump for page after page (and sometimes chapter after chapter) while other people say things like, "That makes sense," and, "I agree". While this element could have easily be halved or cut by two-thirds, some of the pro-sex and not-quite-anti-but-almost-anti-Christian revisionism is interesting considering the time period.
Instead, the flaws are in the overall pacing (once it reaches around the half-way mark, the book nearly forgets it is a quick-paced wartime espionage novel and instead changes gears to become a romp through the tropics with barely a sentence aimed at the all-important downed convoys to be had in the last half) and some horrendously unnecessary racism. In a book about Nazi agents and European war and literal black magic, the book decides to make the "black" part of the word far too racially charged and engages in race-baiting (there is even a line, possibly as a decoy, claiming the good and powerful Euro gods are offended to help swarthy folks).
Even to what degree you can justify the racism of the book with the old hedgehog of "a different time" and "man of his time" and "in the context of its time," I still keep going back to the fact that it is a book of propaganda against Nazis for Skarl's sake. I mean...you have literary Nazis, who are like Nazis but even more evil, and this is a book designed as agit-prop to rally its readers into holding fast against the Nazi hordes, so where in the hell are the (literally, in the context of the novel) damned Nazis? Why attack one of the countries who was actually on the side of helping the US and UK during the war (after the time period of the novel, sure, but it was already extending some visas to Jewish people escaping the Holocaust). A country whose citizens were of a melanin-content that Hitler would have despised. These are the bad guys? Hell.
It also suffers from uneven information dumps. It has no problem dedicating pages to sunburns, or how victims of suicide jeopardize their immortal soul, or the shape of a particular island's geography, or how secret documents are protected from spies, or how religious rituals work but then, for all this "real world" consideration and heavy-handed moralizing, not once does something like a 5-6 hours of time-zone difference and how this would impact sleep-related astral activities get dealt with, or how jet lag would impact dream warriors, or how either of those tie into the greater plot. And its dream logic is problematic, at best, with Wheatley unsure if the dreamscape is the real world with sort of an A.R. game going on or something else entirely.
Of course, things wrap up reallllly conveniently at the end, after pages of diversions and digressions, like the ending was a known thing but Wheatley was aiming for a specific page count goal. The biggest sin here is a chase scene leading up to the climatic confrontation that serves no purpose besides to give false tension and heroic despair at a point in time the novel could easily have progressed to the final battle (and perhaps to give our white heroes a chance to flee from rampaging superstitious black folk, even though on paper the mob is doing the right thing). After that, the real final battle is mostly a philosophical thought experiment, nothing like an earlier sister battle which involved shape-shifting, walls of fire, channeling of the elements, fleeing through dreamscapes, and desperate heroics. That would have been a much more effective end than the one we got.
All it takes is to think back upon how one particular tragic romantic subplot is resolved without passion (both literally for the characters and seemingly for Wheatley), and does little besides to throw some more damnation at black-as-in-African magic, to see how this book brushed near greatness, but fell away from it: too many asides, too much quick summation of some things while other things are driven out, too cat-eating-a-canary smug with its conservative worldview with less care towards what could have been a really clever plot and more given to checking off the "early 20th century British adventure romp" checklist.