First, I'd like to point out (referring to another review) that Middle-Earth really is a Christian world, wherever the small details come from. We can't observe Tolkien's work without adapting to the fact that he was a Christian, and not just in the purely formal sense of the word. Tolkien's texts were always about the everlasting battle of good vs. evil; that is not a Christian invention, but in modern Western literature the connection is easily made.
Well then, about the biography. I think Carpenter did respectable work describing Tolkien's character and way of life. No one could possibly tell us what in the first place made him devote decades for an imaginary world, or where did his ideas come from, but Carpenter casts some light on those matters too: basically what Tolkien did was a scientific process, and his concern for the smallest details is what made Middle-Earth so perfect a creation - simply no faults, no contradictions. Luckily Tolkien also had the gift of storytelling, since that is something you cannot learn. Since I've loved Tolkien's work for years and years, this biography is an essential pack of information. (And I wouldn't have ever heard of The New Shadow without Carpenter, so thanks for that one...)