Or 'letter book', as you suggest.

While I think 'letter book' may be correct, I've never heard of it despite a long career in international organizations, including a stint in a liaison-protocol office. The only term we used was 'correspondence file'.

BTW, "nota", as I know it, is a very high-level exchange. At the diplomatic level, the text is written in the third person. For example: "The Secretary-General salutes the Representative of xx and is pleased to inform him that..." In protocol, we used to distinguish between third-person 'notes' and ordinary 'letters'.

In your case, I suspect they are speaking of important correspondence such as letters of commitment, contracts, etc. - not all the correspondence. So 'letter book', according to the definition you give, would make more sense.

It is also a standard requirement to keep a copy of all outgoing correspondence in chronological order. We used to call it the 'pink file' (in three unrelated organizations that I know of) because back in the days of carbon copies, the pink copy was always saved for this file.



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Note added at 9 hrs (2013-11-22 22:38:02 GMT)
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P.S. My concern about using 'letter book' is that people are not likely to know what it means, though I think it's correct