Biographies of Rimbaud would have details about the book. Apparently 300-500 were printed, but most were not distributed. Book dealers have an encyclopedic memory for the once in a lifetime finds.
From that link:
RIMBAUD, Arthur.
Une Saison en Enfer.
Brussels: Alliance typographique, 1873. (The publisher. An expert would know when and where this press was operating)
12mo******* size
http://en.wiktionary.org/wiki/12mo roughly 5x7 depending on how the binder trims it.
original paper wrappers********* thin vellum-like paper (cooking parchment is close to the texture) that was used to protect the unbound pages from smudges. Typically buyers would have the pages bound themselves to suit their decor, by their favorite binder.
uncut and partially unopened *********** Literally - it's unreadable because the pages are the way they came from the printer, folded from the original larger sheet, but not trimmed because that's the bindery's job. If you cut the pages, you destroy much of the "mint in box" collector attraction.
pp. 53. *** number of pages in the original.
************
It would be a small, unimpressive book of French poetry, perhaps nicely bound but still uncut. I've never seen the page images for it, but it wasn't an "art book".
If it was owned by a collector, they might have had a clamshell case made to protect it, but that would not have been the way it was originally distributed.
