The best set to look out for is the most recent five-volume set
in trade paperback (2006). Two of the previous ten volumes in each book and a serialized (five-part) all-new essay from Alan Dean Foster himself, discussing his writing of the "Star Trek Logs".
In the essays, ADF describes how Judy-Lynn Del-Rey suddenly realized that the adaptations were selling so well they they should spin out the last four episodes to one-per-volume - to extend the book series to ten. The extra material in "Star Trek Log Seven" (about Kirk's former Klingon exchange program roomie, Kumara) makes use of a two-part script that ADF had written for a potential Season Four of TOS.