I'm not even sure how I picked this book up. Perhaps it was the cover (I have a soft spot for the pointy-eared logic elf), and perhaps it was because it was written by A.C. Crispin. Her name does sound vaguely familiar (maybe because I've read her short stories in Tales from Mos Eisley Cantina
I'm quite happy that I did, too. This is a really good story, and one that doesn't ignore or break apart the power trio that was the backbone of the original series. (I'm looking at you, Weinstein! Stranding Spock with Chekov of all people! in the middle of nowhere and ignoring him for nearly half of the book and he's on the cover! Everyone knows the bromance is the key...)
In Yesterday's Son, we find out that Spock's time in Sarpeidon was very productive indeed. An ensign finds a Sarpeidonite cave painting that looks suspiciously like Spock, which leads the Vulcan, with Kirk and McCoy in tow, to the Guardian of Forever in hopes of retrieving his son. They find the boy, of course, but he's no longer a child.