Upon his death, Mallarmé wished all remaining incomplete manuscripts to be burned. A loose-leaf notebook survived which is what Scherer has edited and published as Le "Livre" de Mallarmé (cf. esp. Scherer, 145-52 for a discussion of the manuscript); Scherer also includes a lengthy essay on the possibility of the form Mallarmé's book would have taken based on the evidence that had not been destroyed.
See also my paper "Imposture Book through the Ages" for a discussion of Mallarmé's "Livre" and its possible relevance to Joyce's "Work in Progress."