In terms of copyright, you should be fine. I assume that all these letters were written in the 1940s and never formally published before. If so, then by virtue of being works that were created before 1978 and never published before 2003, the copyright on them in the US has expired. Even if they were, you could likely claim protection from the Fair Use doctrine, since it's for a historical and educational purpose.

That being said, it would be in good taste to try and get the blessing of the authors or (more likely) their surviving descendants.

I don't know the US situation but in most of the world copyright on unpublished materials is held by whoever owns the literary estate. Once it's published it will expire in 70 years. This shows how sick it can get:

https://www.theguardian.com/books/2000/jul/22/poetry.books

The claimed holder of the rights in that case is an American though the materials are in the UK.

Mark Twain successfully embargoed his autobiography for 100 years after his death.

>That being said, it would be in good taste to try and get the blessing of the authors or (more likely) their surviving descendants.

More than just good taste- OP you could get thier permission AND interview them, to provide context around the letters. It would go from a coffee table book to an actual work with transformative properties and add signifgant weight to the publication.

Thank you for the information! And yes when possible I would definitely try to contact any possible relatives.

I’ve actually thought about doing that! It would certainly be awesome

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