I also wanted to ask what you guys what you guys have thought of the other books in the two trilogies? I never read any of them when they originally came out, since I'd never seen Errand of Mercy, but now that I have I was thinking about checking them out.
This often comes up.
The key to the initlal six 2002 books was that they entwined the aired episodes and featured an ensemble cast of familiar faces: the "lower deck" characters, red-shirts, semi-regulars and guest star crewmembers of TOS.
As I suspected (and as was mentioned in "Voyages of Imagination" by Jeff Ayres, 2006), the novel trilogies were inspired by a previous trilogy, "My Brother's Keeper" (1999) by Michael Jan Friedman.
The Janus Gate, a trilogy by LA Graf followed the episode "The Naked Time" and the
Errand of Vengeance trilogy by Kevin Ryan kicks in after "Charlie X" (and expands upon the canonical character of Sam) and "Balance of Terror", but actually incorporates parts of "What Are Little Girls Made Of?" (including adding a "big reveal" re the canonical security guard, Matthews).
It was always obvious that the numerous Klingon plots in Ryan's first trilogy were leading towards "Errand of Mercy". The two instalments of this
new trilogy start immediately after "The Devil in the Dark", so the next episode, in production order, is... "Errand of Mercy".
Update:
I must confess to much preferring the LA Graf trilogy. Kevin Ryan's first trilogy was cleverly constructed, but I found the endless hand-to-hand security squad battles, and who'd be killed off this time, reminded me of the blow-by-blow football game narratives my students at school write. There's not much time for character development when you're hiding behind pallets and firing hand phasers and blasters at each other. But I liked how we knew we were building to Kirk's canonical confrontation with Kor, and we also realise that there are characters who'll lead into ST VI: ie. Gorkon and West.
Ryan's second trilogy has been better fleshed out, and has an interesting subplot concerning an Enterprise crew woman who finds out the security guard she fell pregnant to was a Klingon in disguise. We also meet Sam's father, who continues another plot thread. But of course the first trilogy instalments came out monthly, and these new books have been several years apart. If you were planning to read all six Ryan books in one go, I think you'd get more out of them.