After reading Vanguard - really peaked my interest in TOS - just added these 6 books to my wish list upon your recommendation RonG.
I've always been interested in the overarching themes in TNG & DS9 (wasn't that interested in the standalones) - never really thought there was anything like that in TOS before I read Vanguard. I've also read the two entries about Klingons and Romulans in Seven Deadly Sins and am currently reading Section 31: Cloak. Are there any other books you think would add to the overarching themes in TOS that don't majorally conflict with eachother?
is a definite
IMO! and it does fit in as far as mood / darkness go with both Vanguard
and the Errand
While not TOS per se, I'd also recommend Dayton Ward's In the Name of Honor
, and Dave Stern's Children of Kings