I spent a chunk of my young adult life- in the days before the internet made these things easy- trying to run de Quincey's Suspiria de Profundis to earth. It's a phantom book. De Quincey envisaged it as a sequel to His Confessions of an English Opium Eater but it never quite happened- because de Quincey was a chaotic individual who kept all his unpublished writings in the bath. In so far as it exists it exists in fragments- and what it might have been is beyond reconstruction. At times he drew up lists of projected chapters- with mouth-watering titles like "Count the Leaves in Vallombrosa" and "Oh Sweep away Angel, with Angelic scorn,the dogs that come with Curious Eyes to Gaze"-most of which were either never written or have been lost. In my last post I put up a link to a version which seems to bear de Quincey's own imprimatur but is really little more than a rag-bag for his best and most sustained pieces of doors of perception-busting prose-poetry- including The English Mail Coach which later editors have generally treated as a standalone essay. Since the magnificent cloud-castle that de Quincey envisaged does not and cannot exist this cobbled together assemblage of glorious bits and pieces is probably as good as it gets.
De Quincey is best remembered for The Opium Eater but it's in the Suspiria that his imagination really spreads its wings. It should be better known. Coleridge's Kubla Khan is generally regarded as non plus ultra of this sort of thing but de Quincey goes higher and sustains his flight for longer.