"rebus" (here) stands for "res (Abl.Pl)", meaning (about the) subject (of...).
Literally, "About the cube and its equations". When I translate book titles, I like to check the subject of the book, or at least the subtitle (La genesi del metodo analitico nella teoria delle equazioni cubiche), since titles by definition are compressed, full of information. If you translate the whole book, or even if only parts of it, I do think the Latin title still has the same patina in English as it was in the original, so I recommend to leave it as it is, and give a translation or explanation in the subtitle, just like the original...