Reddit reviews The Road to Middle-Earth: How J.R.R. Tolkien Created a New Mythology

In The Road to Middle Earth it was mentioned that Tolkien admitted he was too kind to his characters, as he cared about them so much. He even let Sam's pony make it back to the Shire after they left him behind at Moria

Read The Road to Middle-earth by Tom Shippey.

A few thoughts:

At one point Lewis and Tolkien were going to write companion novels about space and time. You can see echoes of this in the last chapter of Out of the Silent Planet, the first book in CSL's Space Trilogy when he mentions that space has been cut off from human travel and now any future voyages would be through time. There's also echoes of what might have been in JRRT's Notion Club Papers, which has a time-travel element, but was never published.

In addition, JRRT did not care for the Narnia series because he felt it lacked a coherent theme. However, in the controversial Planet Narnia, Michael Ward posits that CSL actually did have a theme: the medieval view of the planets (The Seven Heavens). There are definitely intriguing arguments made in the book, especially as he combines information from Narnia and the Space Trilogy into his thesis. I wouldn't say it's iron-clad, but if I was still in education, or had the luxury to write papers, this is an area I'd love to explore in depth - specifically the influence of Charles Williams on the evolution of CSL's thought.

If you're interested in aspects of their backgrounds that influenced their worldviews, I would recommend The Discarded Image from CSL (on medieval literature - my favorite CSL book) and The Road to Middle-Earth by Tom Shippey (on the philological undergirding of Middle-Earth). The Humprey Carpenter books are also good (JRRT Letters, Tolkien bio, Inklings bio) as are CSL's letters.

Two really good companion books: Splintered Light and The Road to Middle Earth.

The Children of Hurin is the easiest read and tells the closest to a self-contained story with some -- albeit limited -- expansion from the version in The Silmarillion.

Unfinished Tales is worth it for the history of Galadriel alone but the whole thing is worth a read and gives an interesting look at the scope of Tolkien's thoughts without being as overwhelming as the histories.

I'd also throw in Tom Shippey's The Road to Middle Earth. It's highly readable and quite insightful. Overall a great expansion to Tolkien's own writings.