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).
Or, you could read 'em all and just ignore the bits that don't fit.
All the novels are expected not to contradict canon when they are written, but as TOS movies, TNG, DS9, VOY and ENT were being produced, all manner of large (and many tiny) plot points from past novels and comics would have been trampled by the unfolding "canon" and the other tie-ins. You're never going to find a definitive list of which books remain totally
You risk missing out on some really fun stuff because you're letting others tell you which books are "safe" to read. Are you also going to be scared off by typos? At least one TOS novel ("Mindshadow") misspells "Chekhov" throughout and mixes up Shras (the Andorian ambassador from "Journey to Babel") and Thelev (his deceased, Orion-in-disguise, aide).