Pavane

On the first page of Pavane, Queen Elizabeth I is assassinated. On the second, after the resulting chaos, the Catholic Church regains its medieval authority over Britain. And in the next, the story leaps forward to 1968 (the year the book was published) to a country that's still strangely ancient, but restless. Some alternative histories settle for the shallow fun of merely outlining a what-if scenario, but the deep beauty of Pavane is that it feels lived in, not explained. The story does build, satisfyingly, toward larger, more overtly historical concerns, but it always stays grounded in the grit of individual lives, and especially in some odd, fascinating professions in that parallel world. With the elegance of its writing and the drama of its ideas, and the list of SF stars who love it—William Gibson, Neil Gaiman, China Miéville, George R.R. Martin, among many others—it's incredible that it's such a hard book to find.