The Yesterday Saga
The Yesterday Saga is the retroactively-applied term for a series of Pocket TOS novels written by A.C. Crispin and published by Pocket Books. The term was applied to a  reissue of the two novels, which follow on from the events of .

Novels 
 TOS #11: Yesterday's Son
 TOS #39: Time for Yesterday

"The Yesterday Trilogy" 
More books in the series were planned as early as 1999.  According to an interview given in 2003, a third book was being written "focusing on Zar, the son of Spock and Zarabeth." The basic plot of this unwritten third book would be that  is destroyed and someone has altered the past, forcing Kirk, Spock, and McCoy would have to travel back in time to fix the situation.  According to another interview with Crispin, this would be the first book of a trilogy with the titles of Return to Yesterday, Yesterday's Vulcan, and Yesterday's Destiny, and that Zar would also travel to ancient Vulcan. However, a message on TrekBBS in mid-2004 from Marco Palmieri, then-editor of the Pocket Books line, announced that the project would not be continuing forward, and Crispin's  death ultimately ended any possibility.

See also 
 Worlds Apart – another title retroactively applied by Pocket in 1999.

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Yesterday Saga, The