“It is a curse having an epic temperament in an overcrowded age  devoted to sappy bits!”
A treasure trove of insightful material into the life and writings of Tolkien, but not for everyone. Readers uninterested in Tolkien’s writings need not waste their time.
Where to start? With the negatives, since they’re so few. J. R. R. Tolkien is opinionated, peevish and pedantic. He hated the appellation “professor.”
Among these letters covering most of his adult life, we learn how he viewed his world, his writings, his friends, his religion and his invented languages and history. That is how he saw Middle Earth as history he had discovered as much as created—or, as he would say, sub-created. The letters begin just after publication of The Hobbit and cover the production of Lord of the Rings and the aftermath of its unexpected popularity, and his futile struggle to complete and publish The Silmarillion, which his son Christopher succeeded in publishing five years after his father died.
For those, like myself, who count Tolkein’s works as the gold standard of epic fantasy, these letters give insights only alluded to elsewhere. It’s slow and difficult reading in some cases, partly because context is missing. But the payoff is deeper appreciation of Tolkien’s life and world (real and imagined). We learn the origin of the world, names and characters of the fantasy, and his struggle to keep others from reading alien ideas into the works. Though he admitted (in 1939), “The darkness of the present days has had some effect on it.”
“A most amusing and highly contentious evening, on which (had an outsider eavesdropped) he would have thought it (the Inklings) a meeting of fell enemies hurling deadly insults before drawing their guns.” Sounds like fun.
Now I’ll the only logical thing: re-read The Lord of the Rings. Again.