Economics professor Nathaniel Peaslee suffers a five-year amnesia in 1908 during which he records alien knowledge. After recovery he realizes his mind was swapped with a member of the Great Race of Yith, who exchange consciousnesses across vast time to compile a universal library.
Lovecraft's most temporally ambitious work, spanning hundreds of millions of years, its meditation on the transience of all civilisations gives the cosmic horror an unusually philosophical weight.