The main benefit of this new, expanded, version of the original 1981 selection of The Letters of JRR Tolkien, edited by Humphrey Carpenter and Christopher Tolkien; is that it induced me to "read the whole thing" again, from beginning to end, without skipping (except for a few paragraphs of technical linguistics).
And the main benefit of this thorough re-reading (probably the third) was to realize how much I had forgotten of this indispensable gem of Tolkien's secondary literature. I really ought to have gone through the volume again, long before now; and for making me do it, I am very grateful to the 2023 edition!
If you are going to read any "Tolkien scholarship", then the Letters should be one of your first choices.
But it has to be said that - following a gap of more than forty years between the first and second edition - this new collection is distinctly underwhelming. This because the new edition is in its essentials qualitatively the same as the old edition: qualitatively the same, although quantitatively larger.
The reason behind this sameness is given in Chris Smith's Foreword to the revised edition; which is that the 2023 Letters are, in effect, the Zeroth (i.e. 0th) Edition of the 1981 Letters - that is, the 2023 Letters are the 1981 letters before cuts were made by Carpenter and Tolkien, to bring the volume down to a publishable size.
What we can now say is that the process of cutting the Zeroth edition down to the published 1981 Letters was very well done, because so little of substance was lost. Which (unfortunately) also means that there is not much that has been gained by making available this 40-plus-year-old selection of Letters.
Indeed, I found it hard to locate many of the expansions, short of continually comparing the two editions (which would have destroyed the pleasure of re-reading). There are quite a few new letters, mostly to family members, labelled with a, b, c, etc appended to the numbers, in the correct Chronological position (and without, therefore, disrupting the established Letter numbering scheme).
But many of the 2023 expansions were extra paragraphs added to the 1981-published letters - and the decision was made Not to indicate these expansions in the text (or anywhere) - so that they can only be discovered by a comparison of editions.
Such a lack of editorial explicitness adds to the impression of laziness in preparation of this not-really-new edition of Letters.
My points is that - although the 2023 edition is certainly better than the 1981 edition; the new selection does Not address the core deficiencies of the 1981 selection. I am thinking, in particular, of the lack of any specific reference to Tolkien's psychological and marital difficulties. It was understandable and proper that such references were excluded when Tolkien's children were still alive, but now they have all died it is overdue that these were articulated explicitly in print.
In particular I was disappointed to discover no new letters to cover the 1945-6 period of Tolkien's "nervous breakdown" after he took up the Merton professorship, and during which he was writing The Notion Club Papers.
This was when (apparently) Tolkien and his wife Edith seem to have (informally-) separated for some weeks, and JRRT went off to live with Christopher in an hotel.
I regard this as important in the history of Lord of the Rings, since it was only afterwards that writing of LotR was resumed after a long break.
This crucial period is covered by a distinct gap in the published Letters - whether because none were available, or because they have been excluded, I don't know.
I suppose (eventually....) time will tell. Perhaps this information is being held back for a desperately needed new authorized biography to replace/supplement Carpenter's unsympathetic, indeed semi-hostile, biography of 1977. In the meanwhile, new letter 38a to son Michael from 1940 provides confirmation of the significant and sustained marital problems of Ronald and Edith's middle years - and that these were explicitly known to at least the older boy children.
In sum; the 2023 Letters are in every way better than the 1981 Letters; yet... without really adding anything-much substantive to what was already known.
So that, overall, the 2023 Letters of JRR Tolkien represent a pretty enormous lost-opportunity to publish a genuinely new edition, rather than what is, in effect, an older-than-old edition!