Christopher Tolkien's History of Middle Earth, in twelve volumes, glaringly omits any reference to The Hobbit; apparently because it is 'not part of the legendarium'. John Ratliffe's two books make good the lack. The presentation is tremendously scholarly: there are text footnotes and commentary footnotes, sometimes with footnotes of their own! You need at least two bookmarks if you are to keep track, and a good memory. As to the content, if you are familiar with Tolkien's modus operandi, it will come as no surprise that there were substantial, overlapping, often-incomplete waves of revision; that his cavalier approach to deadlines left his publisher's nerves frazzled; and that he was still reworking it at the end of his life. The Hobbit started off as a free-standing children's story, but it was gradually infected with the matter of Aman & Beleriand, still his private property at that time. Tolkien was an early victim of the internal-coherence obsession that bedevilled Isaac Asimov in his later days, and is a perennial nuisance in Larry Niven's Known Space. The moral is, if you want internal consistence within your oevre, apply it ab initio!