First, as I've said in other comments, I would scour Tolkien's personal letters. Next, read Tolkien's essay "On Fairy Stories" for a general understanding of the importance he placed on creating places like Middle Earth. Then, I would look into a lot of the critical essays done about how his life and work intertwined. Like this one by David Jeffrey and this one by Verlyn Flieger about language being important in his works (he was primarily a philologist). Any essays/books by Tom Shippey would also probably be good sources for you.