I'm almost finished reading The Mask of Fu Manchu, a novel I last had a swipe at circa 1974. I'm not even sure I ever read it properly but some of it did come back to me as I went through it again nearly half a century later.
It works better if you try to forget this book was ever filmed starring Boris Karloff. The plot of the novel was made use of only as the very barest springboard for the MGM thriller's "blood and thunder" potboiler--not even the names of several characters survived intact. Cigar-chomping execs and their lackeys demanded changes lest they "wouldn't go for it in Peoria." (According to film researcher Greg Mank, the entire film was re-shot when the execs rejected the original version.) The entire story wound up being changed into something more like a classic Saturday afternoon serial with fantastically complicated torture devices, while Karloff's Fu was the living embodiment of Yellow Peril nightmares on acid.
Nearing the end of Sax's novel, the real villain of the book seems to this reader to have been loopy, money-mad Edwardian thrill-seeker and insatiably voracious collector Sir Lionel Barton. It's only because the "Devil Doctor" is such an honorable individual that Barton's antics do not cost the lives of his niece and her fiance (hapless, clueless Shan Greville, of whom the good Doctor's daughter comments: "although I love you, Shan, you are really not very clever"). Barton's insane greed for the relics of the Veiled Prophet El Mokanna nearly leads to a huge jihad in various parts of the Near East, one set to make Khartoum look like a weenie-roast.
This is an interesting article about the character of Barton and Rohmer's having based him on Sir Richard Burton (also stated as the inspiration for Conan Doyle's Professor Challenger--hadn't heard of that):