I think I’m probably about two days ahead of you in my dune learning, so this might have typos.
Assuming you are planning to have multiple modules in that lib/ directory at some point you want something like this in your lib/dune:
(libraries <any libraries you rely in in these modules>)
In your bin/dune:
And in your main file you can either refer to Mylib.Letters or do module Letters = Mylib.Letters.
If you want to generate docs with odoc you probably want to set public_name for the library too.
There are a bunch of different naming rules that interact here I think, with the filename=modulename thing plus dune’s stanzas.