There are some two modern books about the Bourbon army of Naples available - in Italian but richly illustrated (and if you understand French or Spanisch you understand quite a lot ... they are:

- L'esercito borbonico dal 1789 al 1815, by Boeri/Crociani (cf. https://www.rodorigoeditore.it/prodotto/lesercito-borbonico-dal-1789-1815/)
- L'esercito borbonico dal 1815 al 1830, by Boeri/Crociani (cf. https://www.dimanoinmano.it/de/cp104776/manualistica/collezionismo/l-esercito-borbonico-dal-1815-al-1830)

Perhaps you may get them via abebooks, as they are the best covering this army.

Greetings from Berlin
Markus Stein