This is definitely one of the more valuable Asimov anthologies for anyone who wants to see his development as a writer.

It contains three stories: the first version of Pebble In the Sky, the first version of The End of Eternity, and the first version of "Belief". Of the three, the last is the least interesting—"Belief" is not, I think, one of Asimov’s better stories in the first place, and the changes are less substantial (the first half of the story is basically the same).

For the early versions of the other two stories, one must admit that the changes were all for the better. It is interesting to see how Asimov has made changes, expanded the plot, added details, and so on, to create a very good novel out of a mildly good novella in each case. Pebble In the Sky sheds an artificial (and embarrassing) stepping out of the proscenium wherein Asimov explains (in the story!) how the story is structured and why. And The End of Eternity gains enormously as we shift our focus onto Harlan and are given his painful love story to deal with.

(By the way, there are people—Asimov among them—who would agree that "Belief" was not improved with revision. The original version of the story was a real downer, and Asimov was rather proud of the fact, since he rarely wrote downers. I would disagree, since I really like the clever approach the hero of "Belief" uses in the final version of the story to solve his problem.)