Forged In Fire
is one of my absolute favorite fill-in-the-gaps Trek novels. It covers implications of the virus, Sulu taking command, Curzon's diplomatic career... so much that we know happened, but we don't know how
it happened. It really ties Enterprise, TOS, and DS9 together in a realistic and satisfying way.
I think that narratively it has its flaws, and the pacing slows waaaaaay down in the third quarter, but as a work of continuity it's beyond reproach. One of those books that goes a long way towards making the Trek Universe feel like one giant place, rather than 5 different series with different interpretations.