^ I don't know that there's anything which would be considered "required viewing" so far as Vanguard goes. The first three books take place after "Where No Man Has Gone Before" but before "The Corbomite Manuever," and therefore don't really play directly off events from any TOS episode. Also, the intent of the series is to add a new context to TOS. Rather than simply offering sequels to episodes or picking up a dangling plot thread and running with it, Vanguard (hopefully) will end up showing how events from the series did not take place in vacuum, and how events in and around the Taurus Reach might impact the Trek tapestry as a whole.
will continue this, but will unfold over a greater period of time. Events from certain TOS episodes will play into the storylines to varying degrees, but more as a means of showing how the "bigger picture" is evolving both within and beyond the Taurus Reach, rather than as direct ties to the book itself (or vice versa).
And then, Mack will come in and blow up whatever's left in Book 5.