1. Before Dishonour - Peter David (2007)
Like DS9 but on a larger scale, after the release of Star Trek Nemesis there really was nothing for the books to worry about in terms of contradicting canon. As such the writers had pretty much free reign, and having been given an inch they took a light year. In terms of sheer scale, perhaps nothing beats the Borg invasion depicted in Star Trek: Destiny, which wiped out dozens of planets and half of Starfleet. Published the year before Destiny however, in terms of emotional impact the one life lost in Before Dishonour is still so much more than a death toll in the billions. The galactic background to which the Star Trek adventures are set could never mean as much as the late, former Captain, then Admiral, Kathryn Janeway (2335 - 2380). Seeking to indulge her curiosity she is assimilated when boarding a crippled Borg Cube, but she never could leave them alone, could she? Becoming a Borg Queen may be the highest honour they could bestow, but despite regaining a few moments of consciousness before the cube was destroyed, even this couldn't save her from a rescue attempt being unsuccessful. Although there are many regular cast members throughout the five Star Trek series, none are as indispensable as the Captains they serve under, particularly those on board Voyager; killing off any regular character may be disheartening, but a Star Trek captain is almost unthinkable. As a great example of the lines becoming blurred, the biggest event in the history of Voyager novels even occurs in a book published under a different series' banner! After something as dramatic as this you just know that from here on in literally anything can happen (unfortunately this also includes the emotional impact of her death being negated thanks to her resurrection by the Q), and 2007 became the year in which Star Trek novels were truly promoted from supplementary adventures, to an irreplaceable storytelling medium.