I ordered this from Amazon because I thought I'd picked it up and read the first half last summer at my in-laws'. In fact this must have been Shippey's other book on Tolkien, The Road to Middle-Earth. Fortuately I'll be at the in-laws' again in two weeks so I can make sure.
Even if I bought it by mistake, this is a very good book, well worth the investment. Shippey argues strongly that Tolkien wanted to achieve the same for the relationship of England with the history of English that Lönnrot did with the Kalevala for Finnish, or that the Grimm brothers (philologists as well as compilers of fairy-tales) did for German. He argues even more strongly, backed by empirical evidence of opinion polls and popular votes (and this was before the BBC Big Read) that Tolkien succeeded as well as any author of the 20th century could do.
Middle Earth, according to Shippey, is an attempt to re-create the mythic background of the English language. Where other writers are content to note that Tolkien nicked the names of the dwarves in The Hobbit from the Elder Edda, Shippey believes that Tolkien is trying to get at the nature of dwarves, elves, etc and to bring back a better understanding of their lost history, which the compilers of the Elder Edda may have known but its readers have certainly forgotten. And he succeeded.
He also argues for a central moral message in the Lord of the Rings, that it is worth trying to do good even if you don't know if you will succeed. Very interesting reflections on the question of whether evil is something that people do (a la Boethius) or has an external objective reality (Manichaeism), which he thinks LOTR debates but leaves unanswered.
There's a lot of other good stuff here, but the most effective for me was a moving look at Tolkien's (non-Middle Earth) short story Leaf by Niggle as autobiography. Niggle is "the sort of painter who can paint leaves better than trees", who is obsessed by the idea of painting one big Tree, so much so that other smaller works get tacked onto the edges of the bigger picture, and he neglects to do the necessary routine work on his own house and garden. It's straightforward enough to read Niggle as Everyman, but Shippey shows quite convincingly that he is also Tolkien.