Roman Numerals I to MM: Liber De Difficillimo Computando Numerum - Arthur Geisert
I paired this with Fun with Roman Numerals and they are a great complement to one another. I would suggest Fun with Roman Numerals first as it has more clear explanations, but this is a fun one to follow up and test newly acquired 'counting' skills. Very delightful, whimsical illustrations make this extra fun!