In addition to the graded readers listed in mondegreen dispenser's answer, the following should also be mentioned:
The Italian publisher Black Cat Cideb has a number of Black Cat Graded Readers for Italian, namely for the levels A2, B1, B2 and C1. (At the time of writing, the only title for C1 is an adaptation of Manzoni's novel I Promessi Sposi.) These books are accompanied by a CD with a recording of the text.
The Lernkrimis ("crime stories for learning") by the German publisher Compact. Most of these books are for the levels A1 to B2, but there is currently one graded reader for level C1 (Il rientro del Padrino). Since they are published for a German-speaking audience, these books have an introduction in German, and some words in the text are translated into German (these words appear in red in the text).
The German publisher dtv takes a different approach: they have published a few books with Italian texts and the German translation on the facing page. A few examples from this bilingual series are: