I was talking to Valerie Aurora and others on Twitter over the last day talking about Ada Lovelace’s letters, and whether there are copies freely available publicly.
The short answer is no.
The long answer is that many/most letters by historical figures are held in private collections. The collectors are often not doing it for the sake of public history: they are either doing it for family history, or collecting letters in the way one might collect artwork, including for monetary value. Access might or might not be granted by the owners to people wanting to use the letters as source material for biographies and so on. Sometimes a volume of letters (or diaries) might be directly published (eg Juliet Barker’s The Brontës: A Life in Letters, which contain excerpts ordered and edited for biographic interest, or Margaret Smith’s The Letters of Charlotte Brontë, which I think is a more complete edition), however the editor of the letters will assert copyright, not unreasonably perhaps given the editing chores and the addition of footnotes, context and so on.
Lovelace’s letters themselves are out of copyright, if I understand correctly (in the US, which is likely to be the strictest, it seems unpublished works authored prior to 1978 are held to the author’s death+70 years rule) but a public domain resource would need to be typed up from the letters themselves rather than from anyone’s existing editions.
It seems what would be most useful would be high definition scans of the letters themselves, without the scanner asserting copyright, hosted by the Internet Archive or similar. Turning these into high quality text transcripts is not trivial, but probably amenable to the efforts of, for example, Distributed Proofreaders, who now provide most of Project Gutenberg‘s new material. Therefore a campaign encouraging people who own collections of historical letters to allow images to be made available is the missing link. Is there such a campaign, or is one needed?