Joris-Karl Huysmans' "The Cathedral" is the third volume of his Durtal tetralogy (preceded by "La-Bas" and "En Route", and followed by "L'Oblat"). I have read that "The Cathedral" has been sold and used as a guide for visitors to Chartres cathedral, and the reader certainly gets a detailed tour of the site through the eyes and thoughts of Durtal, as well as additional commentary by the Abbes Gevrasin and Plomb. The description goes beyond the physical, and includes musings on the meanings and symbolism of most of the ornamentation and decoration of the cathedral (and cathedrals in general). Overviews of the cathedral's history, as well as an overview of the history of gothic architecture in France, are tossed in for good measure.
After the cathedral itself, most of Durtal's attention is focused on the Virgin Mary, to whom he looks for aid with his current spiritual dilemmas. He also supplies the reader with a history of her manifestations in France and descriptions of the sites. Additional sections deal with the religious symbolism of plants, flowers and trees (as Durtal, Gevrasin and Plomb create a hypothetical botanical version of the cathedral), and of animals as well. As always, Durtal also contemplates religious art and the lives of miscellaneous saints.
As can be imagined, this is sometimes tough reading. And the plot itself, you ask? The dramatic tension mostly consists of Durtal agonizing about the "dryness" of his faith, and whether or not this would be helped by making a retreat to the abbey of Solesmes (I won't give away his decision and spoil the ending). Durtal's point of view is both very French and very Catholic.
So is "The Cathedral" worth reading? I think a better question would be if you enjoy the writing of Huysmans. If you read and enjoyed the previous two books, then odds are you will enjoy this as well. If you have followed Durtal's story thus far, then I would recommend this book, if for no other reason than to be able to read final book of the series, "L'Oblat", which is generally a better work. In my opinion, this book is the weakest of the four.
If you are a stranger to the works of Huysmans, then I would suggest you start with "A Rebours" or "La-Bas" instead. If you are interested in Chartres cathedral from a viewpoint of pre-20th century French Catholic mysticism, you might find some enjoyment here. And if you want to know what St. Hildegarde has to say about the spiritual virtues of ferns, then this will be right up your alley. If you have no familiarity or sympathy with the Roman Catholic liturgy or hagiography, then you will be reading with something of a handicap.
"The Cathedral" is not without its pleasures, and some of Huysmans odd detours can be quite fascinating. However, Huysmans continues to reference authors, artists and books that the reader would have either to have lived in 19th century France or been a member of a 16th century religious order to have heard of, as well as an assortment of more or less obscure religious figures and authorities. Also, the dramatic tension of Durtal's spiritual crisis in "The Cathedral" is nowhere near as intense, dramatic and moving as it was in "En Route" - he just seems indecisive here.
The bottom line: "The Cathedral" is not for everybody. Caveat lector!