My argument was that for someone just starting out, like Kat, who wanted something that felt like it could be part of the TV series, I'd probably recommend novels that were really good reads that aren't in conflict with canon continuity. I'd certainly recommend reading some of those novels later. But for what she was looking for it seemed that recommending novels that could literally be part of the original or TNG series would be better.
I still say that TOS didn't really make its own internal continuity a priority -- well, more so than usual for a '60s show, but a lot less so than audiences expect today. For that matter, even TNG was pretty slapdash with its continuity (retconning Data as contraction-less and emotionless in contradiction to his earlier portrayal, retconning in a Cardassian war that contradicted the peacetime Starfleet of the first two seasons, etc.). So if you're looking for something that "could've been part of the show," I think the overall feel and style of the story are more relevant to that than the continuity details. I mean, if it's something really blatant that it breaks the illusion, okay, I can get that; but a lot of the time, the inconsistencies in a novel are pretty subtle, hardly any greater than those that sometimes cropped up within the shows themselves.
For instance, one example I recall is Howard Weinstein's TNG novel Power Hungry. The only inconsistency between that novel and the show's continuity is that the novel portrays planetwide weather control as a practical impossibility, but later episodes like "Sub Rosa" and DS9: "Let He Who Is Without Sin..." portrayed planetary weather control as a routine, long-established reality. Which is the sort of thing that stands out to a detail-oriented mind like mine, but I think most people reading the novel would never even notice the inconsistency, or would shrug it off as a trivial point that didn't affect their enjoyment of the story. Similarly, the main continuity issue with Peter David's DS9: The Siege is that the Rio Grande is destroyed at the end, which we know in retrospect is an impossibility. Otherwise, it fits in quite well with season 1. (There's also the bit with Odo encountering another shapeshifter, but the novel carefully hedges on whether it's a member of Odo's species or just a similar one.)