This was Whittemore's first novel, and it has a perceptibly different tone than the Quartet: darker, proffering human love and cruelty and compassion and madness in equal measure. (Not that the later books are free of cruelty and madness -- the atrocities at Smyrna are one pole of that narrative, as Nanjing is of this one -- but the Quartet is much more about love and compassion in the face of a world which includes cruelty and madness.)
Quin's Shanghai Circus is, nonetheless, full of wonderful things, wonderful people, drunks, pornography, saints, buddhas, a man with a wasabi habit, spies, circuses, mothers, and things shoved up people's butts, all in a wild tangle of storylines that cross decades. Nothing else is like this.