What is a Latin word for account book?

There isn't a single, perfect Latin word for "account book." However, here are some options depending on the specific context and nuance you want to convey:

* Liber rationum: This is the most literal translation, meaning "book of accounts." It's a good general term.

* Codex rationum: This uses "codex" instead of "liber," both meaning "book," but with slightly different connotations. "Codex" suggests a more formal, compiled volume.

* Liber chartarum: This translates to "book of papers" and could be used if the account book is primarily composed of individual documents.

* Liber nummorum: This literally means "book of coins" and is suitable if the book specifically deals with monetary transactions.

You could also use a more general term like liber, codex, or volumen and add a descriptive phrase like "pro rationibus" (for accounts) or "ad rationes" (for accounting).

The best choice will depend on your specific needs and desired level of formality.