Dizionario Di Abbreviature Latini ed Italiani
If anyone has an interest in Latin Paleography and Medieval Latin Abbreviations, one must really consult Adriano Cappelli’s unsurpassed DIZIONARIO DI ABBREVIATURE LATINI ED ITALIANI. Milano, 1912. 529 p., ill. If you can afford it (most copies that I have found for sale range between $36 and $162, scattered mostly across Europe, but occassionally surfacing in American used bookstores), it is definitely worth the purchase.
For online versions, try this 1928 German translation of the Italian on this German site or the original Italian version on a Russian site. A word of caution though, there are a few pages on both of these sites where the pages and scans are incomplete. Between the two sites, the German translation only has two pages missing whereas the Russian site cuts off on pg 428.
