The Letters tells a great deal about who Tom was I think. I saw something a bit different though in the site called Tolkien library. It noted that just as the tale of Roverandom was in answer to the gentle and highly sensitive Michael's losing his tin dog at the beach and though they all searced two days together it was never found; also it was Michael not Priscilla that clamined the Dutch doll Tom Bombadil as his own. His brother John who later became priest it is written actually flushed poor Tom down the toilet but was some how retrieved. And though Tolkien presented a story, a life of Tom Bombadil on its own to his publishers they were not interested. Determined though he managed to have his story , sort of included in LOTR.
But I think it is Goldberry at the pen of our esteemed professor who tells the most about her beloved . In answer to the inquiry of the Hobbits as to just who this almost frightening other worldly personage is she says something like He is Master of the earth wind and hill. She goes on to say that no one has ever caught him in his roamings and living and that he is utterly without fear. She makes a point of noting that he has no desire to dominate , does not need to because in fact he is the Master.
Now, personally , with the knowledge of John Ronald's Christian beliefs I have a sneaking suspicion that he is perhaps unconsciously referred to the first created Man Adam who was not born from flesh and blood but made individually by the Creator or Illuvatar and was breathed the 'spirit' or breath of life into. He alone was given complete dominion over the earth, all its resources and all creatures both then and to come. He then was the Master. But that is my opinion. And because he was just focused on the beauty and wonder of all things other than him he was not interested nor tempted by accruing power as he already had it all. But then of course if the west had lost he too would have met his fate with all the others that were not of Sauron's side.