Emilio Gentile (whose works have the additional quality of being, some times, available in an English translation) has spent a good portion of his career analyzing the early stages of the ambiguous definition of a "fascist ideology" within the environment of what he has defined "national radicalism".

His early works that focus especially on this process - Le origini dell'ideologia fascista (1918-25) [1975, 1996] and Il mito dello stato nuovo dall'antigiolittismo al fascismo [1982, 1999] - provide a good insight into the context of the transition from the ideas of the Italian liberal state to those advocating for a new, albeit often ill defined, political system. Since both are somewhat technical in their perspective, I would suggest pairing them with a general overview of Italian politics during Giolitti's age and the Great War (for instance Bosworth's), unless one is already familiar with the period.

Still with Gentile, and for additional context, his early work on La Voce - Mussolini e La Voce [1976], especially for the influence of Prezzolini's newspaper on Mussolini's (tenuous) attempts at developing his own revisionist approach to Marxism - La Grande Italia [1997] for the affirmation of a new sentiment of Italianismo during the early XX Century - "La nostra sfida alle stelle", Futuristi in politica [2009] for the origins and relative impact of Italian Futurism as a political formation - Mussolini contro Lenin [2017] for a review of Mussolini's "anti-Bolshevism" during 1917-22, mostly taken from the pages of his Popolo d'Italia and for an attempt at a critical examination of how Mussolini's "revolutionarism", or instinct for a national renovation, could adjust to the impact of the Bolshevik revolution.

For a more detailed coverage of Mussolini's relations with the internal currents of early fascism, and for the illustration of the traditional distinction between "party" and "movement", one should check De Felice's Mussolini, especially vol. 1-2-3. As well as De Felice's additional works on the relations between Mussolini, De Ambris and D'Annunzio centered around the "Fiume endeavor" (for context on the latter, one should check Alatri, P. Nitti,D'Annunzio e la questione Adriatica)

In more recent years, specific works have attempted to cover Mussolini's experience as a more or less prominent figure of Italian socialism and his eventual transition to revolutionary interventionism and then to "national" interventionism.

Di Scala, E. ; Gentile, E. - Mussolini 1883-1915, Triumph and transformation of a revolutionary socialist [2016]

For a comprehensive examination of the ongoing social and political landscape across the Great War, I would nonetheless recommend Vivarelli, R. - Storia delle origini del Fascismo and his Il fallimento del liberalismo - the latter especially more focused on the elements of weakness of the Italian liberal system. In English, but much shorter and with a different (and far more "British") perspective on Italian liberalism, Seton-Watson C. - Italy, from liberalism to Fascism, 1870-1925 [1967].