Last night's TORN podcast/video was about this book. It is being significantly enlarged over the first edition. It will feature ~150 totally new letters and ~45 letters expanded from their previously shortened version. Addressing the potential question of how the book is still credited to Humphrey Carpenter despite his being dead for almost 20 years, they said that this is actually the original version that he submitted to the publisher way back in 1980 or whenever this came out. The publisher said at the time they couldn't afford to do a book that long for a limited audience. I guess they think otherwise now!
It was also mentioned that the original numbering of the letters would not be altered by the insertion of the new ones in chronological order. This is a big deal to some people, since sources often cite to "Letters" #123, etc. The inserts will still be in chronological order but will receive a sub number instead of a new number, for example, 8a instead of 9. They mentioned two letters would actually have their numbers swapped, as it has been determined that the one believed to be the later later was actually earlier.
The website Tolkien Collector's Guide (tolkiencollector.com) is doing a guide to the expanded edition:
https://www.tolkienguide.com/modules/ne ... mpost52549
:th