I am not sure there is a single Italian style guide accepted or used by all or most in the publishing industry (I am not sure this is the case for English either, at least if we consider together AmE and BrE). Most publishers have their own Norme editoriali, especially for such stuff as punctuation, dialogues, several types of inverted commas, abbreviations, use of foreign words, upper and lower cases, and also actual linguistic issues.
A well-thought, publicly available example of such norms is the libro di stile of the magazine Internazionale.
For what concerns language in itself, there are several excellent grammars and reference works: one of the most complete and well-regarded is perhaps Italiano by Luca Serianni (with Alberto Castelvecchi) published both by UTET and Garzanti (and mentioned as a reference by the above-mentioned Internazionale). In everyday publishing practice, resorting to Serianni is often useful to settle disagreements.
As regards editing and typesetting (and conventions about everything from internal divisions of the text to relevant ISO norms), a good and widespread reference is Il nuovo manuale di stile by Roberto Lesina, published by Zanichelli.
