Tolkien and The Silmarillion by Clyde Kilby is a hardly-known, slim, minor, but fascinating contribution to the writings about Tolkien; published in the UK in 1977. Its centre is an account of the summer of 1966 which the author spent meeting with the seventy-four year old Tolkien a few times per week, ostensibly to provide him with informed and enthusiastic secretarial assistance to get The Silmarillion ready for publication...
The special value of Kilby's book, and why it still remains well worth reading (if you can get hold of a modestly priced copy) is the 28 page chapter Summer with Tolkien. This I would rate as the best concentrated account I have ever read of what Tolkien was like as a person - in his old age...
So, there are some really good descriptions of Tolkien's appearance, behaviour, manners, mode of speech - and the conversational topics that interested him. Kilby provides a specimen day where he lists, in order, the subjects into-which Tolkien led the conversation:
1. Fan letters, and how T replies to them; 2. Annoyance at an article on T in the Saturday Evening Post; 3. Declaring the birch outside to be his totem tree; 4. How he and Mrs T were annoyed with WH Auden for reportedly making the remark that T's home was 'hideous' in a meeting of the T Society in the US; 5. His dislike of the covers on the Ballantyne paperbacks - and (on publishers more generally) that he was both annoyed and gratified by the Ace paperbacks pirate edition of Lord of the Rings; 6. That he was pleased with the Japanese edition of The Hobbit; 7. Discussing Mrs T's chronic illness; 8. K asks if anyone has asked to write a biography, T says yes and discusses his worries at inner motives being misrepresented; 9. His intention to write a book on the Second Age of Middle Earth, especially Numenor; 10. T showed K some old manuscripts of his work, with evidence of much re-working.
In general; Kilby commented (what the above list confirms) that Tolkien did not seem much interested in working on The Silmarillion!...