My requirements for the novels and comics are these: They don't contradict established canon, they don't contradict each other and they don't contradict 24th century relaunch universe (minor mistakes and discrepancies are all right, we all know that even tv Trek is far from perfect in this aspect).
Well, that limits you mainly to books published over the past decade. With no new Prime-universe canon being produced since ENT ended, there's less risk of contradiction, and the books do generally tend to keep consistent -- though there are certainly exceptions, books that make a choice to disregard the main novel continuity, such as the Crucible
trilogy and The Children of Kings
A number of earlier TOS novels haven't been definitively contradicted yet, but there's no authoritative standard for which ones are "in" and which aren't. A given novel's discrepancies may be irreconcilable for one reader and easily reconcilable for another. Only you can decide for yourself which books you think fit well enough. And if you limit yourself to earlier TOS books that haven't been contradicted, you're kind of ruling out most of the really cool ones.
Naturally, I would probably like to fill the voids between TOS and TAS, TAS and movies and movies themselves, but if there are some nice novels for example from Pike era, then by all means.
There's no clearly defined "void" between TOS and TAS. I think most writers just treat TAS as the fourth year of the five-year mission, though there are one or two who treat it as intermingling with TOS. It's another of those things that come down to individual interpretation.
But it sounds like my TOS books would fit what you're looking for. Ex Machina
is a direct sequel to TMP and addresses some of the character transitions between TOS and TMP and between TMP and TWOK. Mere Anarchy
is a series of six novellas spanning the entire Kirk era from pilots to films, and my installment The Darkness Drops Again
is a followup to Ex Machina
that further fleshes out the era between TMP and TWOK. And Department of Temporal Investigations: Forgotten History
is a novel that spans the TOS era and early post-TMP era, making heavy use of TAS elements, portraying the end of the 5-year mission, and filling in still more of the period after TMP.
series is definitely a must. It's not about the Enterprise
crew for the most part, but it fills in a lot of TOS-era and movie-era backstory and fleshes out the universe from that timeframe a lot more. And it connects to elements of the 24th-century continuity as well.