The alternate Asimovs Book

Asimov, Isaac. The Alternate Asimovs. Doubleday, 1986.
The Alternate Asimovs presents unpublished shorter drafts of two of his most admired novels, Pebble in the Sky and The End of Eternity, and a long short story, "Belief." The original version of Pebble is less focused on the protagonist, whom the revision turns into a truly memorable character. The rewrite of Eternity was much more thorough. Asimov says that the first version did not live up to the promise in the title. I found both versions quite readable. When editors rejected "Belief" because of its downbeat ending, Asimov rewrote the ending (both of which are included here) to make the protagonist a more traditional Asimovian hero. Asimov says he prefers the original ending, but I think his editor was right. Caveat: Asimov is writing for fans, not new readers. He assumes you have read Pebble and Eternity.