(written from a Production point of view)
There are currently two books in the saga written by A.C. Crispin. More books were scheduled, but to date, none have yet been published. 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 Vulcan 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. Unfortunately, a message on TrekBBS in mid-2004 from Marco Palmieri, editor of the Pocket Books line, announced that the project would not be continuing forward. Crispin's 2013 death ultimately prevented the saga from continuing.