I know I'm not the only person here who reads his books. I talked to Kristiina (I think it was Kristiina) about him at one point and she said she thought he might be ENFj. And I see good arguments for that, but I want to claim him for ENFp!
First, for Christmas I got his autobiography, and I got a general feel for him as being ENFp, and also of his dad being ESTj and his mom being INFj. He hardly talked about his sister so no idea there.
When I finished that, I re-read a book of his and I came to a place where he talks about illiteracy rates in the US. He basically says, "Illiteracy is based on A, but it could also be B, and there are arguments for C, plus it could be D because of . . ., also it could be E or F, and we can't rule out G, H, I or J." He never does say what he thinks is the cause of American illiteracy rates. Then he goes on to say that we define illiteracy rates is different than in different countries, and so he goes on to about five or six other ideas for why our illiteracy rates might not be as bad as they look on paper. He does that kind of thing a lot. The book is about the history of the English language in the US, and he's very happy to just speculate on various possible origins of words without ever saying which one is most likely. Over and over again.
He also feels quite content to moralize when something strikes him. There's a place at the end where he's talking about political correctness in language and he says he thinks being careful with what we say in a sense of political correctness (in the US sense of the word) is for the best in certain cases, but that it can't be taken seriously in some other cases. As opposed to his speculation (Ne) he has a very firm idea of what is right and wrong in these cases (Fi).
Those are just examples of what I see as ongoing themes in his book. I'll also say that he comes across strongly as being aristocratic, and he uses that at a source for much of his humor. Like in his first book, where he's travelling from American small town to American small town, he jokes that every woman in every motel he visits has a bee-hive hairdo. He has jokes where he groups people almost constantly. It's over-the-top but it could be based on an aristocratic way of looking at things.