With essays that span the devastating effects of financial inequality and globalization and a new novel on climate change disaster, John Lanchester is becoming the central voice for the end of the world. But such serious business also requires a kind of trickery. It was exceptionally crowded for a weekday afternoon at the British Library as John Lanchester peered into a vitrine containing a curious jewel. It was the final weeks of a sold-out exhibition on Anglo-Saxon kingdoms, and although the show included the oldest surviving copy of the poem Beowulf, the oldest known Latin Bible, and a variety of other literary treasures, these weren’t what the author was most interested in. Instead, Lanchester contemplated a bejeweled golden reading pointer. In the ninth century, its creator, King Alfred, had sixty of them made to accompany copies of his own translation into Old English of a Latin papal text — a kind of premodern marketing campaign. Its most interesting feature is its promotional self-awareness; Lanchester pointed to an inscription on the jewel that read, AELFRED MEC HEHT GEWYRCAN — “Alfred …