Just in case you haven't visited these yet, here's a few useful websites that cover forgeries of the various Italian states.https://stampforgeries.blogspot.com/
- has covered most of the states to date, actively updated. Best site of its kind IMO.http://www.forgeries.info/index.php
- first issues only of all the stateshttp://stampforgeries.com/
- covers several of the states
Sassone's Antichi Stati catalogue (also covers Italy up to 1900) does not cover forgeries in detail, but lots of information as to issue dates, typical usage, bisects, covers, etc that is very helpful. Even more detailed, the Annullamenti (cancellations) catalogue by Sassone is the complete resource on postal markings in this area for the states & early Italy.
Beyond all that, sure there are many more resources in Italian print & online, but to date have found the above sufficient for my needs. Hope that helps.