The End of Eternity is a 1955 science fiction novel by Isaac Asimov with mystery and thriller elements on the subjects of time travel and social engineering. Its ultimate premise is that of a causal loop, a type of temporal paradox in which events and their causes form a loop.

Pebble in the Sky is a science fiction novel by American writer Isaac Asimov, published in 1950. This work is his first novel — parts of the Foundation series had appeared from 1942 onwards in magazines, but Foundation was not published in book form until 1951. The original Foundation books are also a string of linked episodes, whereas this is a complete story involving a single group of characters.