Star Trek: Department of Temporal Investigations/WMG

Shelan will come back in a later book

 * The ground work was laid for this in Watching the Clock. It was suggested, when her Ret-Gone was discovered, that the changes to the timeline were so subtle that actual time travel may not have been involved at all instead using a theoretical piece of Applied Phlebotinum to cancel her out of space-time, and the small differences in history were the result of the universe taking the path of least resistance to fill in the gaps. It turns out it was actually crazy complicated time travel, complex enough that the idea was no one would be willing to try to bring her back because it would be almost impossible to do so without causing more changes. But, if they were to reverse the method that they had thought was originally responsible, pop her back into existence whole sale, then they wouldn't have to worry about damage to the timeline.

Kirk will turn out to be the one who came up with the DTI's anti time travel policy

 * Kirk is regarded both in Trials and Tribble-ations and in Watching the Clock as more or less the Moriarty of time travel. He is also usually shown as something of a speed chess master, particularly in the expanded universe. Also Rule of Funny.
 * Not quite true, as it turns out (Kirk didn't define the DTI policy directly or have official oversight), but Forgotten History does present Kirk as being highly cautious of time travel, speaking in favour of non-interference. Lucsly has to confront the idea that Kirk isn't the demon he's always imagined him to be. There is a bit of a twist regarding Kirk, the DTI and time travel, and another surprise for Lucsly regarding DTI history, but to say more would be spoiling...