Phil,
I gather that Libre is better than Open Office because the latter hasn't had any updates for sometime, but Libre has. I've downloaded Libre, but haven't got around to using it as I have very little need.
If it's just plain letters and no need to include pictures, then WordPad, part of all versions of Windows, does the job simply and just fine. You can change fonts, font sizes, text colours, add bold, italic & underline - it's only if you need to add pictures that you need something with more functionality.
Maurice