If we had every letter
Tolkien ever wrote I still wouldn't consider them canon. Very simply, they were not published by Tolkien for a Once and Future Readership.
I'd say the closest we get here (although not a letter of course) is Tolkien's Nomenclature, which he intended as a guide for multiple translators.
Not only is this made plain in HOME in my opinion, but I see no evidence that the constructed Silmarillion or the constructed Children of
Húrin were ever meant to be taken as canon.
I think we can add
ATB and
RGEO as canon. I add the
Map made by Pauline Baynes
'cause Tolkien himself worked on it (although we know he didn't like some of the illustrations that went with it).
What I
"subtract" (from canon) however is the
revised Foreword to
The Lord of The Rings, as this is not Tolkien as Translator, but Tolkien as Author. The first edition Foreword is canon to me -- and again, here I don't agree with JRRT that it's problematic in any way.
A good argument for posthumously published works as Uncanon, I think. That said, I give great weight to Tolkien's last thoughts about a given subject because we live in time.
And when one is living in time, revision comes after the thing being revised
Moving on. . .
"It may be suggested that whereas my father set great store by consistency at all points with The Lord of the Rings and the Appendices, so little concerning the First Age had appeared in print that he was under far less constraint. I am inclined to think, however, that the primary explanation of these differences lies rather in his writing largely from memory. The histories of the First Age would always remain in a somewhat fluid state so long as they were not fixed in a published state; and he certainly did not have all the relevant manuscripts clearly arranged and set out before him." Christopher Tolkien, Foreword, The
Peoples of
Middle-Earth
To me, this is another way to say that Tolkien,
and quite naturally in my opinion, treated already published work as canon. Or for example, after JRRT realized that he'd already published the word
ros as a
Sindarin word, he wrote "most of this fails" on the text that contained his later, newer idea of
ros as a Beorian word.
In short, for Tolkien, here even a later text mostly failed, due to already published work.
Even when Tolkien just couldn't help himself (between editions of
The Lord of the Rings for example), still, he knew that revising already published works affects the Art of World-building. Not so with revising piles of private papers periodically packed in desk drawers or (possibly) a practical box somewhere.
🐾