I remember the first third of The White Mountains so well--the Watch, the transformation of Jack, Capping and Vagrants, Beanpole--that I was sure I read not only the first book but all three books in John Christopher's trilogy. I finally reread them, quickly because they're children's books, but not with appetite, and now I doubt whether I even finished Mountains, let alone tried the other two: they were completely unfamiliar to me, and very sf-y, and especially the third, The Pool of Fire, was clumsy. A blurb from The Washington Post says that the third is worthy of the first two and "no higher praise is possible." That's an ambiguous remark if ever I've read one.
I've reread his Empty World a few times over the years and I still think it's fine. Also, familiar, and well-worn, whereas a new read as an adult, as these books were, shows a book's hackneyed construction and tropes under a harsh light.