An 'altered' or transformed book. This book, which had never been opened or read and whose signatures remain uncut, had now been 'transfigured' and 'opened'; the unreadable (and unread) sermons on evil have metamorphosed into a 'cathedral', thus eventually finding an unintended but real and related meaning. 'Transfigure' has the dual meaning of changing the outward form or appearance of and to glorify (in the religious context, which was here the author's meaning). [see O.E.D. definitions below (2)] The word 'transfigured' is also defined as 'applied to a solid in which plane faces are substituted for the original solid angles' which is precisely what has been accomplished by the alteration of this book!
The alteration of the book took place on the centenary of its publication in a way which would, no doubt, have astounded the author as would any Freudian interpretation of the book. Buzz Spector in his book The Book Maker's Desire states: 'In 1895 Stéphane Mallarmé noted how the wielding of the paperknife against the 'virginal foldings of the book'(1) was the claim to its possession. But the book of the nineteenth century was unaware of its hymeneal status. Its uncut signatures were simply a result of the bindery technology of the time.' In view of his idea on the erotic aspect of cutting the pages with a paper knife in order to reveal its treasures, this alteration has not necessitated cutting the pages, but has left them in their pristine or uncut state; the alteration has been accomplished without any violation of the book which, considering its subject of evil and sin, was respected.
The folding required for its transformation, did not violate the book and may be reversed. The 'Sackcloth' of the title may be read as buckram, the heavy cotton fabric in which the book is bound. This again has connotations of 'bookness' and the binding is 'transfigured' in that it can no longer be closed as originally intended.
(1) Stéphane Mallarmé. 'The Book: a spiritual instrument', in Mary Ann Caws, ed., Stéphane Mallarmé: Selected Poetry and Prose, New York: New Directions, 1982-83
(2) O.E.D. definitions
1. trans. To alter the figure or appearance of, to change in outward appearance; to transform.
2. trans. Fig. (in allusion to the Transfiguration of Christ): To elevate, glorify, idealize, spiritualize.
Hence transfigured. Applied to a solid in which plane faces are substituted for the original solid angles.